How about this:
Go to your closest cingular store, and sign up for a no contract 49.99 UNL BB Data Only plan as you are supplying your own equipment. You could then compare coverage on your 6710 and the 8700 and see which one gets the best signal. WHile on this plan you could email but have to pay extra per voice minute... OR option 2 is same as above only make it a 39.99 no contract 450 anytime plan with UNL N&W but no M2M and then add the 44.99 BB UNL Plan as a feature... again no contract but allows you to test the network out with no obligation and if you cancel before 3 days are up you are not even responsible for activation charges.
The reason I reccomend the above is that the Prepaid Pay as you Go and Pick Your Plan network is completely different coverage than the Post Paid Network. Just go online to Cingular.com and compare the coverage maps.
Why PM when you can PIN 2035E23E