Its rather simple. Go to the mall and try each out. I did. The keys on the 8700 are terrible. Much, Much worse than on the 72XX or the 71XX. They feel worse - just try typing on them. The 7130 are better and suretype isn't hard to get used to. Its lightyears ahead of T9. It took me about an hour and I am now just as fast typing with suretype as I am with the 7250 which I replaced with the 7130e.
The 7130 is the best BB out there. Just check the reviews