I'm running a Lowrance HDS 8 Gen 1 and HDS 7 Gen 2, both connected to the structure scan black box mounted under the console, so I can see everything they offer (and the same thing on both) in different configurations as I choose. I have the LSS1 TD and I thought it was big, but I believe the LSS 2 (gen 2) is even bigger. The HDS 7/G2 works just fine connected to the LSS 1. I have not "hit" anything with any of my 'ducers but I troll 99.9% of the time, so that lessens the chances of hitting a rock and such, but it's still possible. Both of the Lowrance TD's are mounted on the transom.
The LSS 1 can mount flush on the bottom, especially if you have a stepped transom, the 2d TD is just like any other TD. HB's have an all in one TD, even for side/down scan, and has it's +/-'s from various comments, but everyone loves how easy the HB's are to operate/switch screens. I've read several positive comments on the new Helix line from HB, and have played with one, even made a change for the owner on the chart screen so they are easy to operate. I also have a HB 788 HD ci on my TM so I'm familiar with HB's as well.
You need to go to BPS/Cabelas, wherever and play with 'em, punch some buttons, ask ?'s, and check out youtube for more info. It's a tuff decision to make as it's lots of $$ and you want to get it right the first time.