THe wife and I have At&t, and we pay a little on the high side in my opinion. But we are happy. We both have the iPhone 4-white/black. She has 2gb/month data, I have 200gb, we have 700 min for the both of us, and unlimited text (with that on the family plan comes any any mobile to any mobile unlimited, only on the **family plan 700 min or higher & unlimited text*** that addition is free).
With all that we are at 130$/month. But keep in mind, we do not have a house phone, and we get a 15% or so discount from work, and the iPhones are our only phone bill. If you have any data usage concerns, we only went over once (on the 200mb plan the first month i owned mine) Wifi at the house and work, we rarely use 3G for an extended period of time.
Prices are almost identical as mentioned above with our local wireless coverage Verizon. IDK if its on Sprint yet or not, i don't think so.
The main difference in my opinion is the fact that you cannot talk and use data at the same time on the Verizon iphones/ipads. Meaning; you cannot multi-task out of a call and surf the web to look for something. I do not do that a lot on a day to day basis, but i do it frequently enough to want to keep that ability. The iphone commercials where the fellow makes a restaurant reservation while talking to his wife isn't possible on Verizon.
I have not met anybody with a Verizon iPhone but I can imagine they would love to have that feature.
If you could get it work with T-Mobile as mentioned above if it is unlocked i guess you could use Talk/Data at the same time. No idea.
ALso, I'm sure you have heard things about At&t customer service being bad to horrible. I have a close friend that works for them and he doesn't really have any ability to help me on an individual effort, but he explained to me after a scathing Consumer Reports article that ripped At&t a new one that At&t continues to have the most returning customers over the other Major 4 carriers. That wasn't mention in the Customer Reports review of the the carriers.
HTH