Maybe this will help some
http://www.thehulltruth.com/marine-e...pressions.html
Also on having the HDS 7 for a bigger screen its not a whole lot bigger. My biggest complaint on the "7" was it wasn't a 7 inch screen. Its not even 6 1/2 inches.
All the sub-series of the HDS unit with DSI weren't around when I sold them. They had just come out with HDS and I left around the time Structure Scan arrived. The Elites look like good units. The HDS units should be stronger providing better pictures of the bottom but it's totally a preference and $$ deal.
I played with them every day and loked them but always said for my style of fishing the only way I fork over that much is if fishing pays my bills. Then it becomes a "have-to" for the boat. But until that day there are several that are extremely capable to do what I need.