The HM had an integrated skeg is essentially a longer finer keel edge that stuck further into the water to make it more like a drop skeg, without the ability to remove it. Some people really hated it because they found it hard to maneuver in difficult seas without feeling unstable.

The HS did not have the integrated skeg, but instead had a more "normal" drop skeg.

I actually liked the integrated skeg on the HM and found the boat to be really fun in following seas. I paddled it under the golden gate bridge in fairly steep chop 2-3 foot wind waves with an incoming tide. Great boat.

The HS is a really great boat, but a bit big to be honest for most folks, unless you intend to do a lot of tripping.
–
Keith Wikle