All four of the big wireless operators want to keep their iPhone-loving existing subscribers, as well as win over some new ones.

In an effort to do this, they are each offering trade-in programs that are guaranteeing customers who trade in their older iPhones much better values for a limited time than are currently being offered from many of the third-party trade-in sites, like Gazelle and Nextworth. The difference in trade-in values is particularly enticing for customers with older models of the iPhone.

But with two sizes of the new iPhone to choose from, it's hard to make a decision. In this edition of Ask Maggie, I offer a summary of each of the trade-in programs offered by the nation's largest wireless carriers, and I also dole out some advice for figuring out if the iPhone 6 or the mega-sized iPhone 6 Plus is right for you.

A deal almost too good to be true

Dear Maggie,

I am a 66-year-old grandmother, and I have an iPhone 4 on Verizon Wireless. My contract is already expired and I'm due for an upgrade. I've seen the advertisements about Verizon's trade-in deal that will give me $199 for my old device. I'm worried that I've already missed out on the deal since I didn't preorder it. Am I too late?

The problem is that I simply can't decide if I should get the iPhone 6 or the iPhone 6 Plus. I like my iPhone 4, but I think I'd like something a little bigger.

I use my phone a lot to check email and to keep my calendar. I do a lot of volunteering and am involved in several civic groups. I also use my current iPhone to take pictures of my grandchildren, which I love showing to all my friends and family. I don't do Facebook or anything like that. And in general, I am not very tech savvy at all.

What do you think I should do? And can I still get this trade-in deal from Verizon?

Thank you,

iPhone-toting Nana

Dear iPhone-toting Nana,

You are in luck. It's not too late to take advantage of Verizon's sweet trade-in deal. And believe me, you don't want to miss out on that deal. It's a good one. In fact, all four of the major US carriers are offering terrific trade-in deals for customers with older iPhones that beat the prices you can get from a third-party trade in site, such as Gazelle or Nextworth.

Sarah Tew

But before you run out to your closest Apple or Verizon store, take a deep breath. You still have some time. The deals are good for at least another week. And you need to do your homework on which device is best for you.

Trade-in programs summarized

Verizon Wireless

Verizon is offering guaranteed pricing in the form of store credit or gift cards to customers who trade in older iPhones for the iPhone 6 and the iPhone 6 Plus. It doesn't matter if this iPhone is from Verizon or from another carrier. And you don't need to be a new Verizon customer to take advantage of the promotion.

The promotion offers $200 gift cards for trading in an iPhone 4, 4S, 5 or 5C. Customers with an iPhone 5S will get a gift card of $300. Customers must bring their current iPhone to any Verizon Wireless Communications store or complete an online request by September 30. Once they receive their new iPhone, they have until December 1 to send a receipt and get their gift card. While other websites have reported that Verizon is requiring customers sign up for a two-year contract to get this deal, spokeswoman Debra Lewis clarified and said that is not the case.

"The trade-in promotion doesn't apply to plans," she said. "If you pay full retail price for a phone, and you are an existing unlimited customer, you can keep your unlimited data plan.

AT&T

AT&T's trade-in offer is exactly the same as the offer from Verizon Wireless. And it also expires on September 30.

Again, trading in an iPhone 5s in good condition will fetch you $300, while an iPhone 4, 4s, 5 or 5c will get $200 in store credit. The offer is valid for any new or existing customer so long as you are signing up for AT&T service, renewing service or upgrading on AT&T. The iPhone can be from any carrier. It's not limited exclusively to AT&T iPhones.

To make its deal even sweeter, AT&T is also allowing iPhone 6 customers to take advantage of two other promotions that will give them an additional $100 in AT&T store credit and some money off a new iPad. So on top of the trade-in promotion of $200 or $300, new and existing customers can also get a $100 credit when activating a new line of service with any iPhone on AT&T. And customers who purchase any iPhone on AT&T's early upgrade, no-contract Next plan can get $200 off on a new iPad (The iPad must be purchased with a 2-year wireless agreement.) Each of these additional offers also expires on September 30.

T-Mobile

T-Mobile is offering to match any trade-in quote from its three main carrier competitors -- AT&T, Verizon or Sprint. And if you do a trade in with T-Mobile and see a better offer from AT&T, Verizon or Sprint within seven days, they will beat the offer and give you another $50 on top of that.

What this means until at least September 30 when the AT&T and Verizon offers expire is that you can expect to get about $201 for an iPhone 4, 4s, 5 or 5c as well as $301 for an iPhone 5s in good working condition. T-Mobile hasn't said how long the offer will be available. But since we know the AT&T and Verizon deals expire at the end of the month, it would probably be a good idea to lock in a trade-in with T-Mobile before this date. You can check the value of your iPhone on the T-Mobile website.

Sprint

Sprint already offers its Buyback program that guarantees its trade-in price is lower than its competitors' prices. So just like with the T-Mobile program, you are likely to get the biggest bang for your buck until September 30 when the AT&T and Verizon trade-in promotions end.

Sprint is also offering a special iPhone 6 $200 credit that will allow customers to get a free 16GB iPhone 6, valued at $199.99, when they trade in any used iPhone model. This offer also includes trade-in for iPhones that are even older than the 4-year-old iPhone 4. The phone can be from any carrier and it must be in good working condition. The offer applies to new and existing customers. This deal expires October 31, 2014,

These offers are definitely the best deals around, especially for older models of the iPhone. Third-party trade-in site, Gazelle, is only offering $15 for a Verizon iPhone 4 16GB in good condition. This is a difference of about $185 from the big four carriers. Meanwhile, a 16GB iPhone 5s from Verizon in good condition will get you $265 from Gazelle. That's a difference of about $35 compared to what the carriers are offering as part of their special promotions.

General info about trade-ins

One thing I want to emphasize to you is that even when these carrier deals expire, you can always trade in old devices to a wireless operator or to a third-party site like Gazelle or Nextworth. And it doesn't just have to be an iPhone. You can sell just about any cell phone, tablet or other consumer electronic device online. Also trade-in sites are not the only places to sell old devices. You also have the option to sell your old devices online via sites like eBay. Of course, the big question for most people is which of these places will offer the most convenience and the most value for your particular device.

Apple

Another thing to note is that iPhones in general hold their value more than devices from other manufacturers, such as Samsung or Motorola that are toting the Google Android operating system. But even older iPhones will depreciate over time. This means that the 4-year-old iPhone 4 will likely fetch you considerably less on a conventional trade-in website than last year's iPhone 5S. Also the trade-in values are dependent on many factors.

For instance, a device with less memory, which cost less when you first bought it, will sell for less at a later date than a device with more memory. But there are also other factors that affect pricing, such as the condition of the phone. iPhones that are in good working condition with few if any scratches will get the best prices. And devices with cracked screens, scratches or other faulty bits of hardware will get less.

But as I have noted above, right now the wireless operators are offering terrific buy-back pricing for older phones. In fact, for older phones like the iPhone 4, which debuted more than four years ago, are worth a lot less on third-party sites than what you can get from any of the carriers under these promotions. So you definitely want to make that trade-in happen before September 30.

The last thing to remember is that when you trade in your old device, make sure you delete pictures, contacts and all other information on the device. You want to make sure you wipe the memory and reset it to manufacturer settings. The carrier should do this anyway before it's refurbished and resold to someone else, but you want to make sure all your personal information is deleted before someone else gets it.

Go hands-on with iPhone 6, 6 Plus

Even though these terrific trade-in deals have an expiration date, you don't want to rush your decision between the iPhone 6 and iPhone 6 Plus without even seeing these devices in real life. You mentioned in your question that you are interested in a bigger screen. The good news for you is that each is bigger than your iPhone 4. The iPhone 6 sports a 4.7-inch screen, while the iPhone 6 Plus goes full "phablet" with a 5.5-inch display. So you will likely see a big improvement over what you are currently using.

For the most part, the two devices are identical in functionality and performance. But there are a couple of differences. For instance, the iPhone 6 Plus has better battery life, better screen resolution, and its camera has optical image stabilization, whereas the iPhone 6 does not.

In general, the iPhone 6 Plus will offer you a bit more of a tablet experience. This may make it terrific for checking and sending emails and showing pictures of your grandchildren to your friends. The optical image stabilization will give you crisper and less blurry pictures of your grandchildren. You may also find it more comfortable to read articles and even books on this device than the smaller iPhone 6 and it will most definitely offer a better experience for all of these things over the iPhone 4.

But you may not like the feel of the big 5.5.-inch device in your hand. It's a big device. And it may be difficult for you to use it one-handed. What's more, if you tend to carry your phone around in your pocket or a small purse, it may be simply too big for you. The first time I picked up a Samsung Galaxy Note, which is an equivalent size to the iPhone 6 Plus, I remember feeling like I was in one of those novelty joke shops where they sell the really big pencils and oversize chairs.

If the oversize nature of the iPhone 6 Plus is overkill for you, you will likely be very happy with the smaller iPhone 6. You will definitely notice the difference. And you will not want to go back to your older iPhone 4 or any other device of that size again.

The bottom line

My advice to you is to go to an Apple store or stop by one of the carrier stores and see the devices in person before you make your final decision. Once you are secure with your decision, trade-in your iPhone 4 to take advantage of the $200 offer. And you can put that money toward the cost of the iPhone 6 or iPhone 6 Plus.

I wouldn't worry about getting the device in your hands right away. If there is a long line at the store, just go home and place your order online. If you're already at the store and there is either no line or you don't mind waiting, you can purchase it right there. Don't be upset if the size or configuration you want is not available at the store. Most of the carriers are allowing customers to order the devices in the store to take advantage of the trade-in pricing and then they will receive their new devices in a couple of weeks depending on availability.

Keep in mind that if you want the iPhone 6 Plus, most of the wireless operators don't have those in stock yet. Apple stores probably will. But given this is one of the biggest iPhone launches ever, there's likely to be a steady stream of customer traffic at Apple stores throughout the weekend. Again don't fret if you can't get the device in your hands this weekend or even before the September 30 expiration of the trade-in deals. So long as you order your new phone by the time the offer expires, you should be fine.

I hope this advice was helpful. Good luck!