The biggest difference is the HD has an ethernet port, which makes it easy to link to other HB units so they can share some info, like waypoints. Probably not a useful feature in your current setup. Also has a different base map, I believe, but neither is likely to have much pond detail.
SI/DI and better 2d sonar can help your fishing for sure. You're able to see the weed beds better, find the edges of them, see structure off to the sides, etc. Great fishing tools.
08 Skeeter SL210, F225 Yamaha four stroke, 798CI SI NVB, 997C SI
(formerly owned an 1197C SI NVB Combo)