I'm not currently running HB, but a significant % of Lowrance users in the past have claimed SI doesn't work worth a crap on the TM - i.e. images smear from turning the TM. My perspective is that this is true inasmuch as the sudden/sharp turn of the TM would of course smear the images but a gradual extended and constant turn (long enough to get a picture) or straight run should return as good an image as that on the transom.
I have the equipment (Lowrance) to equip both my TM and stern with down/side scan, although not mounted at the moment. The front set-up was used when I got it along with the head unit and complete rigging so it didn't cost me a $400 difference.
My opinion (affordable as it is) is that if you like the side imaging capability, it should work within the parameters I explained, as long as mounted as level as possible when in use. Whether the $ difference is worth it is your determination. If you like the feature and make use of it often, I'd go for it, as I'm thinking you're not the type that changes electronics every whip-stitch.
I don't know if you can network the front and back on those units to show back info on the front screen, or not, which would be an option - although your info would be 18' old when seeing it up front.
It's one of those "if you have it you don't have to use it - if you don't have it, you CAN'T use it"