Have you looked at the 9700 section and read the threads there? That should give you a good idea of the 9700. Also look at the Blackberry and T-Mo websites for specs and features. You can compare the two side by side on T-Mo I think.
As for why you should switch, well that is a personal thing that only you can decide.
Why are you looking at the BB? What features do you want and need? What do you not like about the G1?