Apple is now finally selling SIM-free, unlocked iPhone 6 and iPhone 6 Plus through its online and brick and mortar stores. The new models, which come nearly four months after the two devices launched, are available in all storage configurations, starting at $649 and $749, respectively.

Apple makes things slightly complicated for those looking to purchase an unlocked iPhone 6 or iPhone 6 Plus, as the company is advertising two "Unlocked and contract-free" models, for each of the two devices, on its site. The model that is available starting today is labeled "SIM-free", while the other one bears T-Mobile's logo. Be careful which one you buy, if you plan on using it in US or abroad.

First off, here are the model numbers for the two unlocked iPhone 6 and iPhone 6 Plus models that Apple is selling:

iPhone 6:

T-Mobile: A1549

SIM-free: A1586

iPhone 6 Plus:

T-Mobile: A1522

SIM-free: A1524

The so-called SIM-free iPhone 6 and iPhone 6 Plus are also sold by Apple in Europe and other parts of the globe, like China. That means that they can be bought in US, and used successfully on carrier networks abroad, with full support for 4G LTE -- the last bit is extremely important, if you care about fast data speeds.

Also, these models are the only ones that support China's implementation of 4G LTE (known as TD-LTE), on all bands used in the country. They also work on CDMA networks, but come without a nano-SIM. For international use, as you can see, they are ideal choices. (My iPhone 6 Plus, purchased in Europe, has the A1524 model number. Here's a tip -- you could save some money by importing an iPhone 6 or iPhone 6 Plus from US to Europe, as long as no VAT has to be paid -- otherwise, the cost is pretty much the same.)

However, the SIM-free iPhone 6 and iPhone 6 Plus do not support 4G LTE on all major US carrier networks. They can be used on, say, Sprint and US Cellular, but not AT&T, T-Mobile and Verizon. This is a major reason why you should look at the T-Mobile counterparts, if you plan on getting an unlocked iPhone 6 or iPhone 6 Plus to use in US.

The T-Mobile iPhone 6 and iPhone 6 Plus do not support TD-LTE, according to the information provided by Apple on its site. They, however, come with a nano-SIM, and, most importantly, can be activated on all carrier networks in US. So, theoretically at least, you should be able to use the T-Mobile models successfully on AT&T, Sprint, T-Mobile and Verizon.

In case you are wondering, below is the complete pricing breakdown. (The T-Mobile and SIM-free models cost the same.)

Unlocked iPhone 6:

16 GB: $649

64 GB: $749

128 GB: $849

Unlocked iPhone 6 Plus:

16 GB: $749

64 GB: $849

128 GB: $949

Will you be buying an unlocked iPhone 6 or iPhone 6 Plus?