Interesting you should ask.
I've had a 7100t on T-Mobile for a year and just ordered an 8700c (of course Cingular). Partly because I wanted a full keyboard, partly because Cingular offers discounts through my School and T-Mo doesn't and partly because I like to browse a lot and wanted Edge.
It should arrive tomorrow (Monday) but, as is usually the case, only after I ordered did I delve into the threads here about additional data charges (I'm not on a BES). So given the above advantages I would be concerned about two things:
1. If you're not on a BES and like to browse and use bbWeather and GoogleLocal, etc., there is a decent chance you'll be hit with excess data charges. Not good;
2. Cingular, at least now, does NOT use the newer RIM method which does away with the BWC site. Whereas with the 7100t I can respond to e-mails with any of my configured e-mail accounts, with Cingular you still have the website and the problem that you're going to have only one reply address (unless you use 3rd party software).
Given that there is a 30-day return period I'll play with the 8700c and see if the positives outweigh the negatives. Perhaps by then the RIM lawsuit thingie will be settled and/or TMo will have the 8700.
Good Luck with your decision.