FWIW, I'm still with AT&T, for the next 5 months, but I also signed up with T-Mo, as its the only 3G carrier that will work with the n900. I'm currently using the n900 on AT&T with its Edge speeds until my contract ends, and its fine for just email, and browsing forums, VoIP calls. I dont think it will handle streaming a Sling feed, if/when the 900 gets Sling support.
3G coverage is about the same in my area, in the North East, but the big draw is getting a voice and data plan for $40 on T-Mo. AT&T reception was never an issue for me with a Motorola Q, again in the NE, from NC to RI. I'm seeing similar results in the same area on T-Mo, if anyone is considering switching. However I remember reading something about AT&T's lower frequency bands (850) being better for reception, due to better building/wall penetration. Again, I had no issues in older city buildings where people on VZ, with BBerries, did have issues. Or basically people on 2G networks/phones.