In terms of speed, AT&T retains a slight edge. Apple engineers have found a way to harness AT&T’s HSDPA bandwidth and apply it to the iPhone 4S. However, while AT&T refers to this standard as “4G,” it is more like 3G+, and improvements in speed are minimal. Verizon has consistently tested as the most reliable network overall, and is thus often the carrier of choice for customers whose main concern is sheer workhorse dependability.
In addition, if users of the new iPhone on any carrier have cell reception problems, there are excellent cell phone signal booster options that work extremely well with iPhones.
Good luck choosing your carrier.