A blackberry 7900? I don't know of that model.
My colleagues have the 8700 and they seem to like it quite a bit. Definitely a step up from their 7290, since they can now connect to the faster Cingular EDGE network rather than GPRS. I opted for the 7130c because it's more phone-like (it's a shame I didn't get the Pearl that came out a few months after I got my 7130c - that thing is beautiful). I wasn't really fond of the 8800. I didn't like the keyboard as that on the 8700. And while it's thinner, it's actually longer too.