According to GSMWorld (here), Cingular operates both 1900 MHz and 850 MHz towers whereas T-Mobile operates only 1900 MHz towers. If you were to choose a Cingular SIM there IS a small chance you may end up in an area with only 850 MHz coverage and will therefore be unable to place calls with your handset.

If your phone doesn't support 850 MHz then you might want to stick with T-Mobile. If you want to go with Cingular, you could always pick up a an 850 MHz handset once here in the states. If you're into gaming, the Nokia N-Gage QD phone is dual-band 1900/850MHz and only $99 at GameStop with games on clearance at 50% off.