I have the option to trade-in a phone to tmobile. They are offering me $350 for my sprint iphone 5s. I can buy an iphone 5s from ebay for 350-400 which will be unlocked for any carrier (so technically I'll be getting an unlocked iphone 5s for free to $50) or I can buy the iphone 6 from tmobile for 650 (~700 after taxes) which comes out to paying about 300 extra after getting my trade in credit. What should I do?