You should check the coverage maps and ask people in your area what kind of signal they get. You can also go to the Options -> Mobile Network screen on a Pearl and manually register on the T-Mobile or AT&T network (or any other GSM carrier in range), then go to Options -> Status and check the signal strength. It should show up even if your phone can't register. Keep in mind that the number displayed is negative, so -70 dBm is a much stronger signal than -100 dBm.
I have T-Mobile, and it has horrible coverage in my area. 2-year contracts suck.