I have had both (but not a BlackBerry on Cingular, though) and I found that T-Mobile is cheaper but whoever offers the best coverage is who you should go with.
Tmo no doubt. I did my homework before I switched from Sprint (Treo 650) about 3 weeks ago. Cingular's pricing is outragous. I could never afford a plan for my needs with them.
But as stated above, cheaper is not always better. More importantly, check the coverage in your area.