You can buy the T-mobile version as long as you have a nano SIM already activated it should work, if not, Verizon will give you a hard time activating it. I have an iphone 4 right now cuz I sold my iPhone 6 a couple weeks ago, so I opted for purchasing the Verizon version. Another option would be going to the actual apple store, they will allow you to purchase one outright.
I decided to open a new account only because I rather have a Verizon iPhone instead of the T-mobile. Also because I live like 2 hours away from the nearest apple store, so I rather just wait for it to get here while in the confort of my home.