The reason I left Sprint for my current carrier, Verizon, had a lot to do with the overall ratings on the carriers in a Consumer Reports from earlier this year. I found consistently low ratings for Sprint on data, text, voice quality, call drops—nearly every category. There was a score system in which Sprint was the only one of the four traditional carriers with a score below 60. (It's not that the the other three—which also include AT&T and T-Mobile—were over 80. But Sprint's low performance couldn't be ignored.) With the exception of perceived value, which probably has more to do with people taking their entire bill (including folding into their monthly price the installment payments for a phone not paid off), Verizon had consistent No. 1 ratings. (Well, I think I now recall that AT&T may have been rated No. 1 on the data reliability.)
This awakened me to the fact that, with choosing to move from a basic phone to a smartphone, I should not remain with Sprint and, instead, should go with a different carrier. I was out of contract with Sprint. So, I was free to move.
I bought an iPhone 6 Plus at an Apple Store. It's Unlocked. So, if anything comes up that would lead to me switching from Verizon to AT&T or T-Mobile, I will have the freedom to be able to do that. My decision to go with Verizon has nothing to do with rejecting AT&T or T-Mobile. This had to do with making a choice and knowing that it wasn't wise for me to remain with Sprint. But, with AT&T having bought DirecTV, I feel AT&T would have to do plenty more to persuade me to move over to AT&T. (I have an employee discount which currently makes Verizon the best for me personally.)