One important caveat: While the T-Mobile iPhone 6 does not require a contract, it doesnt come unlocked either.
The T-Mobile iPhone is contract-free and can be used with the GSM carrier of your choice internationally, but comes preconfigured for use with T-Mobile and needs to be activated with T-Mobile in the United States, an Apple retail support representative wrote. To use the iPhone with another carrier in the United States, you will want to contact T-Mobile for your available options or wait for the unlocked version of the iPhone to be released.
From
http://www.ibtimes.com/how-pre-order-iphone-6-when-where-purchase-apples-smartphone-1686108