Despite a thicker spine, the SDFK has a slightly narrower primary grind than the HDFK, which will give it a slight advantage in cutting ability in some applications.
The HDFK is meaningfully lighter (it is obvious to me in the hand and in use) and has some additional hand grips that may make it a slightly better camp and carry knife for some people, though they're both pretty reasonable stout woods knives. And, although the SDFK is practically indestructible, the HDFK has an even more durable tip for prying. Although the SDFK probably wins in an overall prying contest.
Both of them, if it ever fails for any reason other than intentionally breaking it I will repair, replace or refund. (Broken tips are repointed, broken scales are replaced with the material of my choosing). You can't break these knives without tools, and I dare you to try. And if they break with the use of tools they are covered unless you were doing it to break it on purpose.
The HDFK is perfectly balanced for its intended purpose. The SDFK is a good well balanced knife but not quite as perfect as the HDFK.
The HDFK represents some of our finest work and an important collaboration with Lorien.
The SDFK is less complicated to produce and is subsequently a better value. It has been a while since we produced the HDFK and there have been a few years in there with some really gnarly inflation so direct cost comparisons are not obvious to make, but the SDFK is a little bit better bang for your buck.