For most data centers I can forget 3G. For some of them you'll be lucky to get voice service unless you have verizon. The only "normal" places I consistently have trouble is in the taller buildings like 1 Penn Plaza. Above 35 or something like that there's hardly any service from any provider so it's probably not a 1900 vs 800 thing. It's been a couple of years so it could have changed in that building. It can vary widely even a few feet one direction or another in any building really.
Vierizon definitely has the edge when it comes to reception inside buildings in my opinion but there are few enough places where ATT simply fails that I can easily deal with it. Since the 2.2 software my dropped calls have gone way way down. Until 2.2 I was considering getting a verizon flip for voice.