Don't Switch to AT&T if You're Happy Where You Are
AT&T is an excellent carrier and you may be frustrated with T-Mobile or Verizon right now due to a billing issue or another common problem, but remember that the grass always looks greener on the other side of the fence.
If you are happy with your current service overall, it's a good idea to stick with what is working unless you can save more with a promotion or discount. This is especially true if you like the added freebies that T-Mobile offers or the free access to GoGo that you get with your T-Mobile service.
Compare all the things that are working on your plan with how they work for a friend. If you can't find a friend that is using AT&T, you might be in a location where there is a better option.