Better PrePaid Service: T-Mobile or AT&T (Two Day Use)

Going to the US for two days and would like to use my unlocked Bold with an AT&T or T-Mobile SIM card.

What is the better option to go with? The SIM card itself is cheap to purchase, but what carrier will have a cheaper rate service based on myself going for two days?

Not going to be making many calls, looking mroe at their data rates over the two days. Don't need the 3G that I believe only AT&T would offer because I don't plan on browsing while there.

Thanks.