Whether you're on a prepaid plan or paying full price because your contract isn't up yet, here are some things to consider when buying an iPhone 6 contract-free. There are actually more models of the iPhone that are available unlocked and will work with multiple carriers. See which one is right for you.
iPhone 6 models for different carriers
There are three different models each of the iPhone 6 and 6 Plus that work with the carriers in the US. There is a CDMA model for Verizon, another for GSM carriers such as AT&T, T-Mobile, and Cricket, and the last is for Sprint, C-Spire, and US Cellular. It's a little confusing, because the Verizon and AT&T/T-Mobile models have the same model number with a CDMA or GSM suffix. Get the details on which carriers and LTE bands the different models support.
The contract-free iPhone Apple sells online and in their retail stores is unlocked and comes with a T-Mobile SIM, since T-Mobile doesn't subsidize phones anymore. This model is exactly the same as the AT&T version, but unlocked. Furthermore, the Verizon model shares the same hardware as the AT&T/T-Mobile model, but with CDMA radios enabled. All Verizon LTE phones, including iPhones, are sold unlocked (as required by the FCC), and will work on GSM carriers, such as AT&T or T-Mobile.
Put simply, the Verizon iPhone 6 will work on AT&T, T-Mobile, or other GSM carriers, but the unlocked T-Mobile model will not work on Verizon. The Sprint model has different hardware altogether and is designed to work only on Sprint, C-Spire, or US Cellular.
Verizon or unlocked GSM model?
Several prepaid plans run on AT&T or T-Mobile's GSM networks, so the contract-free T-Mobile iPhone will work with them. It's relatively easy to buy the contract-free T-Mobile model from Apple online or in-store. However, it is also possible to go to an Apple store and purchase a Verizon iPhone 'device-only' (and pay full price for it). I've heard it's possible purchase an iPhone at full price online from Verizon or at a store if you have an existing Verizon account and are ineligible for an upgrade, but I've never done in personally.
It may be worthwhile to buy the Verizon model for a few reasons: there are a lot of Verizon customers out there it can be sold to down the line, if you ever decide to switch to Verizon you won't need to buy a new phone, and Verizon is the only carrier that officially (if not publicly) allows the use of tablet plans in a smartphone.
I looked at trade-in values of iPhones a while ago, and the unlocked model actually fetched a higher price than Verizon models on eBay and Gazelle. The Verizon model is unlocked though, and most sellers probably don't realize this and market it as such. Currently, the unlocked iPhone 5s fetches a bit more than the Verizon model for a trade-in on Amazon.
The standard contract-free iPhone will work just fine for most people looking to buy a full-priced iPhone to use on a GSM carrier. It's also easy to acquire directly from Apple. However, the Verizon model will also work on GSM carriers in addition to Verizon, and may be a better option for some people.