You've opened Pandora's box with that one. Make sure even before you start looking make a list of things you need in a fish finder, then make a list of wants. Then, compare both units. And yes, they are almost the same.
Personally I've used the Hbird 1100SI series, 900SI series, 700 series, and 500 series in action on the water. I've played with a demo HDS, but haven't seen the HDS on the water... YET. I've used a bunch of the "older" lowrance LMS 520c, 110c, ect. on the water for most of last years season. Now that lowrance is releasing its new down+side imaging its pretty much the same as the hbirds as they too now have down imaging standard on their SI units.
The sonar capabilities of the units will be roughly the same. The down and side imaging should also NOT be purchased for a 5'' unit as you wont be able to see enough detail. At the end of the day, these units are so similar it pretty much comes down to price. Also don't let the "HD" thing fool you, its pretty much just a marketing gimmick, theres higher resolution on the birds.
Heres one on the HDS 5 and 700
http://www.walleyecentral.com/forums/showthread.php?t=160441
Heres an ok discussion between the HDS8 and 900 series humminbird.
http://www.walleyecentral.com/forums/showthread.php?t=162672
What am I buying for this season? I have absolutely no idea yet and I've been researching the units for about 3 months straight.