When it comes to which carrier to use for your smartphone there are two main considerations to take into account: cost and coverage.
Although landing an AT&T or Verizon account may yield impressive coverage, the cost may be difficult for some to swing every month. This fact is even more pertinent with the wider availability of free and ubiquitous Wi-Fi networks. The quality of the coverage also matters in relation to your work and home life. Once again, AT&T or T-Mobile could be amazing, but if you get zero bars at home, their service is useless.
Recently, I spent some time using Ting (Ting.com) in the US. Never heard of 'em? No worries, as I tested them out for a few weeks using the HTC 8XT.