I've owned a 7100 model, the 8700 and now the 8800. I love the 8800 for these reasons over the 7100 model --8800 --full keypad,slim phone even with the full keypad, a lot faster load times then the 7100,media options, you can have an sd card with the 8800 for music/pictures/etc. The trackball with the 8800 is different...but I like it now better than the scroll wheel the older models have. In my opinion I would go with the 8800. I have had mine for a few months now and love it.
I've blogged on the 8800 at my site.
http://btkuhn.blogspot.com
Be glad to answer any additional questions you may have.