I've used both the 7290 and the 7100r. As I mainly use the blackberry for emails first, phone second, I'm am anxiously waiting for the 8700. The 7100 has an amazing screen, but as other posters have said, I miss the little things from the 7200 series. The suretype works well for the most part, but the words that use the same keys are annoying. The main thing I miss though, is the shortcuts off of the main screen.
In short, if you are mainly an email user and phone user second, then I'd wait for the 8700, but if you just need to read emails and use the phone primarily, then I'd recommend the 7100. But, Stinsonddog is right, if you are just using a headset, then I guess it doesn't matter.