navionic, the 798 unit is a Side Imaging unit and the 788ci HD DI is the new Down Imaging unit.
The Side Imaging technology is more expensive to support with hardware and software so that is why it costs more.
The new 788ci HD DI unit does not do Side Imaging, it uses the imaging frequencies that the 898, 998, and 1198 Side Imaging units have (455/800) to produce Down Imaging and it only has one 2D frequency (200 khz). The transducer for that unit and the other DI units is new.
There is another 788 model also that ONLY does 2D sonar in the 83/200 khz frequency.
The 798 uses only the 455 khz frequency for Side Imaing and Down Imaging plus it uses 83/200 khz for 2D sonar.
