My husband was overseas last week for work and he has T-Mobile. He called T-Mobile to see what his options were. Basically, he could text, using data and pay $.25/minute or he could buy a daily pass that was $5/day or he could pay $35 for 10 day pass of unlimited data.
The $5/day requires you to call every day you need it, so he just went with the $35 option because he didn't want another thing to do in the mornings.
I don't know what your options are with T-Mobile prepaid, but you could also buy an international SIM card when you arrive. Just do your research if you decide to go that route. It's easy to get ripped off.