As noted, Cingular will take the AT&T over, so buy which ever will lock you into the best rate now.

Switched from AT&T to T-Moblie a couple of years ago because of terrible customer service.
Switched back because AT&T has the only acceptable level of service in my particular part of town.

Have been happy with them since.

I have the 600 minute, 2 line National plan which included unlimited minutes to/from anothe AT&T subscriber. Since both of my sons are AT&T, and my wife is on my plan, talk is cheap all over the country.