AT&T may have widespread coverage, but that means jack stuff if you always live in an urban environment, especially because their signal can't penetrate deep into buildings like Verizon's can. Verizon has better signal strength for sure, I always lose signal inside basements and some areas because of AT&T's stuff signal, and I still have AT&T.
Also AT&T has probably the worst customer service. They pushed out a 4.0.4 android update some time ago in the winter and it was an absolute trainwreck, almost everyone with a Galaxy S2 with that update was reporting just constant sleep of death problems and yet nothing from AT&T.