The last time I researched this was 3 years ago, so I'm sure my info is out of date. So rather than describing why I decided what I did then, I'll just suggest which criteria to consider...
- If there are particular phone features important to you, those may dictate which service you choose. For example, if it's important to you to have an Apple iPhone, then you have only one carrier to choose from. Some carriers (such as Verizon) will disable features in the phone as a way of forcing you to use higher-cost services. For example, they may disable the feature that allows a person to connect a phone to their computer to load ringtones into the phone, instead forcing you to use the phone as an Internet browser to get the ringtones you want.
- AT&T has a reputation for being slower with Internet access and such than other carriers. If you don't plan to use your phone for Internet access, then it doesn't matter. But if you do, keep this in mind.
- If you have any friends with cell phone service on the carriers you are evaluating, ask them to bring their phones to your house and see what the reception is like. I dropped AT&T and moved to Verizon because AT&T reception in my own yard was horrid, even outdoors. Really horrid. So try to test each carrier you're considering in your own yard.