On T-Mobile here too using a 7290. The only places I've had coverage problems is deep inside large office buildings, & parts of Jamaica Queens. I have the $45.99 1000 minutes & free nights & weekends for voice, & $19.99 BIS add-on for data. When I went to Canada back in April, I added the $19.99 International Blackberry Data Roaming option.
Note: if you go this route, you need both add-ons. I had a novice rep mistakenly remove my BIS add-on thinking it was a replacement, & blew away my data connectivity in the process. Thankfully a RIM tech was able to recover my account & settings.
I have no data experience with Cingular. I found their voice coverage was on par with T-Mobile in the metro area when I was researching both networks. In the end, cost won out for me also.