It depends on location more than anything. Verizon has the best overall coverage of any carrier, but AT&T is not too far behind in a lot of areas. The carriers' coverage maps will help give you an idea of how they'll work for you where you live and work.
Also, if you're a Windows Phone user, AT&T is preferable because you can use almost any unlocked phone. If you're not a Windows Phone user, or are willing to switch, it doesn't really matter.