depends where you are.

if you're in the US, Verizon is the way to go

 

you have to take into account how the reception is in your area, where you'll be using the phone, and where you'll most likely be traveling to a lot. around my area here, Verizon is the way to go. T-mobile sucks balls in my area. lousy reception and their phones are kinda junky in our market. for the most part, Verizon will get better service throughout the US. keep in mind that as with all services, Verizon and T-Mobile will still have their dead spots. Do your studies, and that does not mean going into a cell store and asking the associates. they'll tell you whatever to make the sale. when you go out with your friends, take note as to who seems to be getting better service than the other. and don't just use one friend as a rep for that company because the hardware has a lot to do with signal strength too. if both are the same, then go T-Mobile. it's cheaper. =)