Well I have Sprint and Iphone 4S and don't have any problem with the phone connecting! Sprint is knocked by everyone but there is places ATT is suppose to have service and it drops calls frequently! My Sprint phones I've had always had reception everywhere I go. Sprint has roaming agreements with Verizon so you're covered almost everywhere as it is! If you use your a lot of data Sprint is the way to go! Call customer care and have them go through the settings if your having issues like that.
It all depends on your coverage where you live. I was on sprint with no issues for 2 years. Their speeds are very usable when you factor in that they are unlimited. I had to switch to AT&T when sprint had several towers go down for over 6 months and refused to repair them. Up until that point I was fine with them. It all depends on the service coverage where YOU are.