The latest iPhones are just around the corner, and that means some Apple fans may be looking to sell their old devices.

Apple is expected to unveil the iPhone 6S and iPhone 6S Plus on Wednesday during an event in San Francisco. The new iPhones will likely offer only incremental updates to the popular line of smartphone. That may not be enough to get owners of last year's models to upgrade, but plenty of iPhone 5 and iPhone 5S owners will be looking forward to the next version. Because Apple devices hold their value well, wireless subscribers with old iPhones can expect to get a good chunk of change if they sell their used devices.

Dozens of websites offer easy trade-in programs for old smartphones. Some retailers also offer customers the option to sell their used devices. Even wireless carriers are making it easy for customers to trade in their iPhones and upgrade to new ones. Which option is right for you? That's the question I answer in this edition of Ask Maggie.

Dear Maggie,

I'm planning to trade in my old iPhone 5S for a new iPhone 6S or iPhone 6S Plus when it comes out this month. I want to sell my old phone, but I'm not sure which is the best way to do it.

What are the benefits or downsides of using a trade-in site like Gazelle over going to a retailer like Best Buy or simply trading it in with Verizon? Where am I likely to get the best deal?

Thanks,

iPhone Upgrader

Dear iPhone Upgrader,

Wireless carriers are moving toward eliminating phone subsidies and requiring customers to pay the full price of their devices. It makes sense for subscribers to think strategically about how they resell their old phones. This is especially true for the iPhone, which traditionally has started at $650.

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Luckily for you, there is no shortage of options for selling your device. As you note, there are dozens of websites dedicated to accepting trade-ins. Amazon, Gazelle and Nextworth are among the most popular.

Several retailers also take trade-ins, such as Target, Walmart and Best Buy. Apple offers a device reuse and recycling program, which can get you credit toward the purchase of a new device. Then there are the wireless carriers themselves, which also offer buy-back programs.

Which is the best option for trading in your old phone? Unfortunately, there's no simple answer.

Getting the best price is a top priority, but some consumers might sacrifice a few dollars for convenience. You'll want to feel the resale process went smoothly, without any surprises. Here are some tips that can help you work through the confusion.

Note: Before you trade in your iPhone, make sure you erase all your personal data. CNET can show you how to make sure your device is wiped clean before you sell it.

What to consider

Price

The whole point of selling your old iPhone is to get money to help you pay for a new iPhone 6S or 6S Plus. Naturally, you would rather sell your device to the place that will pay the most for it. Sites such as Amazon, Gazelle and Nextworth consistently offer aggressive pricing for trade-ins. The prices aren't as high as you'd likely get if you sold the device yourself via Craigslist or eBay, but these sites offer a convenient and safe way to sell your phone. They also tend to offer better prices than many of the wireless carriers or retailers, including Apple.

You should always compare pricing among these websites and also look at the prices carriers or retailers are offering. Sometimes Nextworth's pricing beats Gazelle's, and Amazon may offer more than Nextworth. (Also, Nextworth runs some in-store programs for retailers, such as Target. So you can get the same deal in a store as well.) To find which site offers you the best deal, do your homework. Each of these sites enables you to select the model, carrier and condition of your phone to get an estimated price. You can also look for special promotions.

For example, right now Gazelle is offering a Best Price Guarantee on iPhone 6, 6 Plus and 5S trade-ins until Wednesday. As part of this promotion, it's promising to beat the trade-in values offered by Apple and carriers AT&T, Verizon, Sprint and T-Mobile. The offer covers all storage variations and colors. If you find a better deal from a carrier or from Apple, you can send proof of the offer to Gazelle, and it'll match that deal.

Keep in mind this promotion is limited to beating offers from wireless carriers and Apple, which is not yet accepting trade-ins for the iPhone 6 and iPhone 6 Plus. The offer won't match or beat prices from sites such as Amazon or Nextworth nor will it beat offers from retailers like Target and Best Buy.

AT&T and Verizon say they are offering up to $300 for old iPhones, but the real value you will get depends on the model and condition of your device.

Here is a quick look at the price difference on the trade-in value for a used iPhone 5S in good condition from Verizon with 16GB of memory. Gazelle is offering $201, but at Nextworth it's $170. At Apple it's $165, and at AT&T $135.

Cash or credit

Another thing you may want to consider when trading in your device is how you'll be paid for it. Most of the retailers offer store credit as opposed to cash. Wireless operators also don't give you cash when you trade in your device. Verizon's current promotion offers customers store credit or a Verizon Wireless gift card. If you trade in a device at a Verizon Wireless store, you'll receive instant credit. If you mail in your old device, the credit will show up on your bill within one to two billing cycles. Either the credit or gift card can be used to pay for service or accessories.

Receiving credit instead of cash may be fine if you shop at that retailer often, or you can use it to pay down your future phone bill. But nothing beats cash. Gazelle and Nextworth will send you a check for the value of your device. Nextworth offers other payment options as well. For instance, you can get a PayPal credit that can be accessed the same day the transaction is complete.

The fine print

It's important to read the fine print on these deals. Make sure you know what, if any, conditions there are on your trade-in. Gazelle's promotion won't beat every trade-in price offered. Some of the most attractive carrier promotions may only apply to new customers or customers adding a line of service to their plans. Also, the maximum amount offered may not apply to your device if it's not in pristine condition.

Beware of leasing programs

Sprint and T-Mobile offer attractive leasing programs. Sprint's iPhone Forever plan lets customers lease an iPhone for $15 per month for 22 months with the option to upgrade to the latest iPhone whenever they want. T-Mobile's program is similar. It also offers an iPhone for $15 a month and lets customers trade in their old iPhones for a new one when it comes out. These deals sound great because $15 a month is $12 a month cheaper than financing a new iPhone through AT&T or Verizon.

The problem with a leasing agreement is that you don't actually own your device. When the leasing term ends, you are left with nothing. The phone isn't yours. You can't sell it to help defray the cost of a new device. In other words, you will always be paying an extra fee to Sprint or T-Mobile each month for the use of your phone.

This may not be an issue if you plan to upgrade every year. With a leasing program though, you can't keep your old iPhone as a backup in case you lose or break your new device and you can't pass your old iPhone on to someone else in your family.

Convenience

Time is money. For many people, standing in line at an Apple store or another retailer to trade in and buy another device is simply not an option. For others, they're willing to sacrifice time at the mall so they don't have to wait days or weeks for a device to be delivered and a trade-in to be processed.

The decision to mail your old iPhone to a website like Amazon, Gazelle or Nextworth or to walk into a store where you can drop off your old phone and get a new one on the spot is a personal one. But when you're considering your options, you should factor that into the equation.

What should you do?

You need to do your homework to find the best price for your used iPhone. Check the popular trade-in websites, as well as the carriers and retailers. Remember that pricing on these devices changes frequently, so check before committing to any of them.

You're likely to get a better price from a trade-in site. I would go with either Amazon, Gazelle or Nextworth. I prefer ordering things online, so I am comfortable sending in an old device and waiting for a new one to arrive. If waiting isn't an option for you, you can always look into Nextworth's in-store program at Target. But remember, you'll receive a Target gift card as payment.

I would probably avoid trade-ins with a carrier, since they tend to offer the least amount of money for an old device. I'd also avoid leasing programs. Unless you plan to upgrade to a new phone every year, it makes sense to own your device. That way you can sell it or give it to a family member when you're ready for the next upgrade. But remember that as contract plans go away, subscribers will be responsible for the full price of their new iPhones.

I hope this advice was helpful. And good luck!

Ask Maggie is an advice column that answers readers' wireless and broadband questions. If you have a question, I'd love to hear from you. Please send me an e-mail at maggie dot reardon at cbs dot com. And please put "Ask Maggie" in the subject header. You can also follow me on Facebook on my Ask Maggie page.