I went from Sprint to T-Mobile to AT&T. Sprint when we lived in KC and moved to Iowa. T-Mobile when I had to start paying for my own phone. That experiment lasted 4 months. **** coverage in my home in Ankeny, and NO covereage West of Jordan Creek, even on the interstate. So I switched to AT&T since it was GSM so I could transfer the phones I owned.
US Cellular, Sprint, and Verizon are on the CDMA network. I have a pretty sweet deal on AT&T and are pretty happy with them except for the other thread I started about them not giving me credit for my 2nd phone on a BOGO offer. "Should" be resolved in the next two days, that will be the only reason I'll leave them I'm guessing. (But this could be an issue more with the Best Buy salesperson and myself not reading the details closer.)
With AT&T we have 3 lines, 15 GB/month, and carryover data, and our total is around $150. A DirecTV buddy of mine was telling me that AT&T/DirecTV are looking to buy US Cellular since all their installers had to get AT&T phones when they merged and most can't get coverage in the smaller communities. But that was a while ago and I haven't heard anymore about that.
My summary is that if you can afford what you have, stay with what you have. Even if it is for a "free" phone deal with Sprint or T-Mobile. And know that your only option if you go with T-Mobile will be to go to AT&T with that phone.