The difference is that 770 is really a long term goer, not a sudden boom. You could compare it with 'pocketpc' in the search - another stable long term platform. But considering that 770 has a single vendor and is a new product line, it is not doing that bad at all! (Some other search terms would fare less well, 'palm lifedrive' for example... or 'pepperpad'...)

The difference between umpc and tablet PC is that a tablet PC appears to be just a laptop with a few extra usability features. So with a tablet PC you get all the benefits of conventional laptops - you could run latest 3d games and stuff - you have a full keyboard, etc. I don't know if tablet PCs have been such a flop as people claim, the concept is interesting but I suppose sales volumes are not so high, so prices tend to be on the higher side compared to conventional laptops.