Well I don't have sprint, but many of my co-workers do have it. In the past two months I have gone from t-Mobile to AT&T to Verizon (currently on), and I can tell you that I seriously regret leaving AT&T.
Here is the reasoning for each carrier:
Sprint:
As someone already mentioned it's a sinking ship. If you were to buy the EVO LTE of SG3, they charge you an additional $10 a month for the use of LTE (which does not exist).
Yes they are cheap and unlimited....but at speeds of 0.4Mbps down....you will never reach even a 2 or 3GB plan.
T-Mobile:
I had a similar situation as sprint....great price, but unreliable network (at best). Every time I entered a building, any building/store, I was lucky to make a phone call.....and you could forget about data.
Verizon:
Proces are high yes....but You do get great service. I have no issues with phone or data service, provided I am outside my office building (and in particular, my office). While I am at the office I get nothing, and this is primarily the reason why I want to return to AT&T. Overall VZW is a great network. If you don't live in a mayor city, chances are you will get great signal with VZW.
AT&T:
For it's great....I really like the 4 tier network they have, where if LTE is not present, you fall back to HSPA+ network. LTE is not official here in South Florida, and I was getting an average 4-6mbps down just about everywhere I went. in the areas where LTW was present (testing) I could get 20-21 mbps down.
Over all AT&T and VZW have the best networks available right now. Down the road say a year or two Sprint and T-Mobile might be up to snuff, you can always switch back to them, but I doubt that they will catch up to VZW or AT&T any time soon.
My choice is AT&T, and right now I am trying to find a way back to them without having to pay penalties and/or fees.
Talk to friends in your area on other networks and see what they say.