Buy an unlocked GSM phone, and activate it on T-mobile. Or, keep the one you have, if it can handle US frequencies.
Of the four national US carries (small regional carriers typically piggy-back on the big nationals), only AT&T and T-mobile are GSM. Verizon and Sprint use CDMA.
Over ten years ago I dropped AT&T after their network became too saturated, and became pretty much unusable. From what I hear, things haven't changed.
Verizon and AT&T have the largest network and best, fastest coverages; but if you're moving to a large, populated, city, T-mobile's coverage will probably be as best as the bigger guys. Out in less-populated areas, far away from the civilization, Verizon's going to be only game in town; sometimes it's AT&T.
Sprint falls somewhere in the middle between Verizon/AT&T, and T-mobile, who is the smallest, but I think they're the most friendly to people who prefer to use their own, unlocked phone, and have very low tolerance for US cell carrier B.S. They even used to have discounted plans for people who bring their own unlocked phones, but I don't think they do that anymore. They do have "pre-paid" plans, which seem to be a bit cheaper.
There's no such thing as an unlocked CDMA phone, so with Verizon or Sprint you have to buy one from them, when you buy service. Verizon is notorious for feature-castrating their phones. It's been my experience that "Bluetooth" on Verizon's castrated phones only means a wireless headphone. That's it. No bluetooth file transfer/browsing, no other Bluetooth profiles. If you want to load your own MP3 ringtone, you can only get them on the phone by buying them from Verizon. Sometimes, I heard a rumor that some Verizon phones let you configure an MP3 ringtone that you've transferred over USB, but, it's been my experience that the UI on Verizon's phones do not let you select an uploaded/copied ringtone.
I've been happy on T-mobile for the last ~10 years. They don't care what phone I use, I just pop in a SIM, and off I go. I finally decided to get a data plan as a back-up for my wired broadband, since I telecommute. Set it up, then twiddled a bit with my phone, and had it set up tethering without any issues. From what I heard, if you want to tether with the bigger carriers, you're likely end up getting charged extra, on top of paying for the data. Utter bullshit. From what I've heard, they've been getting bitch-slapped recently, on that account, because, supposedly they're not allowed to do that anymore, as a condition for buying some recently-auctioned wireless spectrum. Whatever, I don't care.
As far as prices go, the differences between the carriers are pretty much negligible. The only other thing is: T-mobile, themselves, does not sell Iphones; but if you get an unlocked GSM one, shouldn't be too difficult to activate it. Verizon and AT&T are the primary carriers of Iphone in the US. I think Sprint might be selling them too, though.