So I want to get an iPhone 6S. I'm 1 year into a 2-year AT&T, but I have the option to steal an upgrade from someone else. I would then have an iPhone 6 (128 GB) to sell/trade-in/whatever. I'm a bit overwhelmed with all the different options, between renewing my contract, switching to AT&T next, or switching to T-Mobile. I'll want to upgrade every year at this point. What's my best option?