Rev, it can make a difference which transducer you use. The larger HDSi transducers have a longer Si element in them that form a narrower sonar beam (front-to-back) and so can show better resolution. The problem with using this on a smaller display unit like the 798 is that not everyone can see the difference in resolution. Those who really study their Si sonar could but not everyone can (failing eyesight makes a difference too). Now if you made a sonar recording or a screen snapshot and viewed that on a larger display, such as your computer, then most will be able to tell the difference.
Also, the transom mounted version of the Compact Side Imaging (CSi) transducer that is normally used with the 798 unit; has less shielding than does the transom mounted HDSi transducer. If your boat has a lot of radiated RF energy (from the motor, trolling motor or other source); the HDSi transducer could make a difference there as well. Problem is that most will not know that they have an RF interference problem until after they have their unit on their boat and are using it. [The trolling motor mounted version of the CSi transducer does have this additional shielding.]