On your phone (the N900) I can assure you that T-Mobile will have better support the AT&T

So the quality of AT&Ts coverage is not really an issue.

It has been a year+ since I was in the US but yeah T-Mobiles 3g is still growing and has a few issues on the east coast anyway.

But it was inproving rapidly. Dont expect much coverage in the boondocks though. Not yet.