Thing is, in some areas, Cingular uses the 850 Mhz frequency band, which tends to reach farther and is generally better at penetrating things like walls, parking garages, and tunnels than the 1900 Mhz PCS band used by T-Mo, VZW, and Sprint. That could explain why you get better reception with Cingular in the bathroom.
IMO, Cingular has better phones than T-Mobile right now. I haven't had a bad experience with Cingular CS yet, but maybe I've been lucky all these years. They are usually very, very helpful. We have had a couple billing problems, though. So... just keep that in mind. But we have never been overcharged.
Cingular has just updated their rate plans at 12 AM tonight, but the price structure isn't really what people had been predicting. Take a look at the site if you're interested.