I actually just switched my kid from tmobile to at&t

t-mobile has no signal inside high school building. Only weak signal on the top floor (3rd floor). So I changed his phone to at&t. Just reported back from school that he now has signal everywhere at school.  

Of course, at&t is more expensive than t-mobile.  We are paying $100 a month for 3GB data, including paying monthly for his phone.

Have heard that t-mobile has improved their network significantly recently. But in our case, at&t's coverage gives us peace of mind, knowing that if he needs to call in emergency, the call is going to go through.

So that's something to consider...