If they're both healthy and in their primes, I'd go with Henderson. Much of the supposition will be based on MMA math, but that's the way it goes with dream fights that span several time periods.
Henderson's most inconsistent fights where at MW, where he was cutting weight and it was effecting his cardio. He wouldn't have the concern at 205.
Chuck, for all his KO-power-from-all-four-limbs was still vulnerable at his peak, as illustrated in the first rampage fight [before Chuck's obvious decline]. Using rampage as the common opponent, you can get some idea that sways an arguement. It's MMA math, so it doesn't prove anything, but I think it brings perspective.
Very simply, I do not have rampage anywhere close to Henderson's level. At jackson's absolute best, rampage won a controversial decision over Dan, and had several factors in his favor with the fight: It was Henderson's first fight back under full unified rules and UFC pacing, as well as in the cage, rather than the ring; while rampage had a tune up fight and the [second] win against Liddell to acclimatize himself to UFC standards. Henderson's wife was nearly 10 months pregnant when the fight occurred, and was home on another contintent. Mentally, that's going to take a toll. Finally, he was having the same issues with his back that have plagued him for much of his career.
That's the MMA math side of things: rampage at his peak handily beat Chuck at his peak, but couldn't do much more with a clearly-less-than peak Henderson than eke out a controversial decision.
In terms of skill, I think they're pretty similar. A natural LHW, Chuck is much bigger, with a huge reach advantage, and gets the nod there. I have Chuck's overall striking rated superior to Dan in it's variety, but not necessarily in power. I have Dan's chin rated better than Chucks, throughout their careers. I have Dan's grappling superior to Chucks; in both groundgame and clinch, but give the edge to Liddell in submission defense, although I'm pretty confident that any action going on at the mat would be GnP rather submission grappling.
It would have been a great fight, but I just feel Dan has more edges where it counts, and would have ended up landing that big ohr that would have heralded the decline of Chuck much earlier than it would have occurred naturally.