Double check sure Cingular will give you better service. They were using the same towers all across California until Cingular bought AT&T Wireless. I think they sold all their towers to T-Mo since AT&T had the 850MHz frequency. I think they can still roam on T-mo towers (not totally sure about that though) so you probably have access to both the 1900 and 850 towers with Cingular.

I get pretty good coverage in L.A. with T-mo though. GSM frequencies don't penetrate buildings as well, so the only times i really have trouble are when I'm deep inside a building. My Dash seems to get better coverage and hold a signal a bit better than my MDA does, which might be largely due to my fingers not blocking the antenna when typing!