The 798 has a wider screen (640 x 640) vs the 797 (640 x 480), has 2 SD crd slots instead of 1, and has 3 "soft keys" that you can use to set "preferred" views to flip between, rather than scrolling through all available views using the "view" button. The 798 can also be upgraded to Down Imaging, while the 797 cannot.

My advice: of the two, get the 798.