I have Cingular GSM (and have had Verizon and Sprint in South Florida) and in terms of ability to get a signal, even Cingular's TDMA beats those two hands down.
T-mobile coverage in South Florida is provided by TDMA & GSM (and in many cases they lease Cingular/AT&Ts towers)
Honestly, the only real hitch I think you'll have switching from Tmobile is the fact that Cingular has a totally screwed up package for Instant Message/SMS & Data. TMobile's TDMA coverage was better than Cingular's TDMA in some areas, but, their GSM seems about the same.
Hopefully Cingular will have a deal for data that is a little more attractive for users of the iPhone (or whatever Cisco will allow it to be called)
And Cingular is supporting Edge out of the gate -- but not supporting 3G. EDGE is a bit slower, but, still pretty darn useful for instant messaging and emails that you would want to compose on a phone. 3G would allow video to be streamed to your phone without problem.
Edge in South Florida is 135kb/sec in most areas, but, goes down to 70kb/sec based on distance and 'ruralness'
3G in South Florida (if you can get them to sell you 3G) is 400kb/sec - 700kb/sec. Then of course there's CingularConnect which has something like 4000 wifi hotspots nationwide -- many through Wayport (a hotspot provider). Doesn't include McDonalds wifi though...