I personally would choose based on the carrier who gives you the coverage you require. Bottom line is you need to stay connected. If they both offer dependable coverage in the areas you travel then the choice becomes more difficult.
I can't comment on the 8700 since I've not used one but it appears to be a well thought out device. I did receive a 7130e this week and am VERY pleased. This is my first BB and I have been told I am very clear and easy to understand when I make calls and what I hear on my end is just as good. It's much better than the Samsung i730 I tried. On the email side I'm also very impressed and am getting used to SureType very quickly.
Good luck with your decision.