I have heard that T-Mobile is the best iPhone carrier right now. I need to be able to use the internet at the same time as the phone, so I think Verizon and Sprint are out?
I'll agree with what the others have said: T-Mobile is great,
IF the coverage in your area is good and you don't plan on traveling much to places where it's bad. Unfortunately, there's a LOT of bad. They are also very, very far behind in not just their LTE deployments, but in overlaying HSPA (3G/"FauxG").
That's really what sank them for me. I really, really wanted to switch to T-Mobile because my iPhone 5 is unlocked, can handle AWS and I could save myself close to $35 a month. But the coverage maps are keeping me away. No LTE where I live and work. Places I travel frequently have spotty coverage, and some areas have only 2G coverage still, even though I'm not in the boondocks. It would be a HUGE step down, and it's not worth the lower cost when I often
need to be reachable, and a good, fast data connection would be nice to have.
Your best bet is to
check the coverage map. Not just where you live, but where you work and any place you think you might be visiting in the future. If it's all solid coverage, then it might be worth trying out. If there's a lot of 2G or no coverage, stay away.
And yes, Sprint and Verizon are definitely out if you want voice and data at the same time. Even worse for Verizon: they're going to be launching Voice over LTE soon (which is good), but
with no circuit switched fallback (which is really, really bad). that means if you're on a voice call in LTE and your signal is weak,
the call will drop, even if there's strong 3G signal in that same area. Verizon's CDMA isn't capable of packet-switched calls, so it can't directly take a handoff from VoLTE.
By contrast: AT&T and T-Mobile could deploy circuit-switched fallback, but they don't really have to. Their 3G is capable of supporting the same packet-switched calls as LTE can, so a switch down shouldn't be a problem. Of course, it would be nice if AT&T started rolling out VoLTE soon...