No responses on this, so perhaps this vicinity is of interest only to me. Just in case, here are my observations.
I switched from Cingular to T-Mobile on Saturday, and from my unlocked BB 7290 running 4.1 to a new BB Pearl 8100 running 4.2. I'll post my Pearl observations on one of the review threads. Bottom line, it's the best phone I've ever owned and I prefer it (slightly) to my wife's 8700. Either way, I'm a Crackberry addict.
20% Edge to T-Mobile
T-Mobile - 600 Anytime Minutes, Unlimited BB, IM, and Data, $60 per month. Upgrade phones every 12 months (not that I plan to for a LONG time.
Ciingular - 450 Anytime Minutes w/Rollover, Unlimited BB, 200 Text/IM, Unlimited Data, $75 per month. Upgrade phones every 18 months.
* Overall, Slight advantage Cingular, though sound interference (static and skips) is more prevalent on the Cingular network with the 7290.
* EDGE vs. GPRS, Advantage to T-Mobile EDGE
For whatever reason, the 8100 holds EDGE reception better than the 7290 holds GPRS. Both do fairly well with that. On the 8100, my web browser performance is much better on EDGE than the 7290 on GPRS. Noticeably faster with fewer delays and not a single timeout.
* At home, Scotts Valley CA, Skypark area. Edge to T-Mobile
Cingular (7290) - About 3 bars inside, GPRS. varied from 2-4 bars all around SV. No drops. Mediocre voice reception quality.
T-Mobile (8100) - About 3 bars inside, EDGE. varied from 3-5 bars all around SV. No drops. Good voice reception quality. My house is RIGHT next to a T-Mobile tower. performance is very good.
* Highway 17, Santa Cruz to Hwy 85. Slight edge to T-Mobile.
Both varied from 2-5 bars througout the route. Cingular dropped one call on the SC side of the Santa Cruz mountains. Both avoided the Verizon dead spot between Pasa Tiempo and Santa Cruz. Sound reception quality was slightly better on T-Mobile.
Cingular (7290) - 3 one bars dips in the mountains, 1 near SC
T-Mobile (8100) - 2 one bar dips in the mountain, 1 near SC, one near Lark Ave exit.
* Highway 85, Hwy 17 to Sunnyvale. Slight edge to Cingular.
Both varied from 3-5 bars througout the route. No calls dropped.
Cingular (7290) - 2 two bar dips. Slightly better average signal strength.
T-Mobile (8100) - 1 one bar dip and 2 two bar dips.
* At Work, inside, N Mary and Central. Big edge to Cingular
Cingular (7290) - 2 bars, mostly gets GPRS lock. Decent performance.
T-Mobile (8100) - 1-2 bars, intermittent drops. Mostly gets edge lock. I'm between two towers and my office will move to a better coverage area in 6 months.
* Santa Cruz area, Graham Hill Road from Felton to SC, Big edge to Cingular
Cingular (7290) - 1-4 bars.
T-Mobile (8100) - 0-1 bars until the cemetary outside of SC, mostly looking for network. Essentially no coverage in this area. I fear that this will be the biggest drawback wih T-Mobile. Their coverage, while good in mainstream areas, is still lacking in more remote areas.