I have the older 797 which has a 5" screen. The 797 is missing some useful features, like the dual SD slots, which the 798 has, so now it really gets down to how big of a screen will fit into your budget (798, 997, 1197).

Images taken on my 797 will show much more detail if the images are copied to an SD card and displayed on my computer. Picking out individual fish on the small screen can be difficult, but once you learn to use it you'll be looking for structure that will hold fish and won't be worried about seeing the fish on SI.

With SI you'll be able to find structure you never knew existed. Large screen or small you'll get better information of what's below the surface with SI than you ever would with traditional down looking sonar.