I have a 24" Stainless HBAR 1:8" twist on mine. I find it swings only slightly slower than the 20", but just feels "right" for me.
If you are going to be stalking, i would recommend the shorter barrel, just not to get caught up in brush. If you're going to be still hunting/varminting rested or with a bipod, you could choose the 24" and be ok.
As for velocity, mine runs "slow" compared to what one would expect (not saying yours would). I don't know if that's a function of an over-generous chamber, if the SS bbl has more bullet resistance/friction than a chrome lined one, or it just has a bore that needs a good polishing. Of course, the barrel isn't what anybody would consider broken-in yet, so that may change.
Hornady 75grn TAP only runs ~2720 fps in my 24", which is only ~100fps more than in a 16" (chronoed same day at range).
XM193 runs about 125-150fps faster from the 24" than the 16", giving around 3350fps avg in the 24" (again chronoed same day).
Accuracy in mine is currently un-spectacular, but very very consistent. Groups (of any ammo or handload thats not-Wolf) run 1.5"-1.8"@100yds, with wolf going into 3"-3.5". All groups measured OOTO, not CTC. This is off the bench with the barrel rested and NOT free floated, which is hardly ideal. The consistency is a good sign, and I expect groups to tighten a bit with a FF tube and a non-military/heinous trigger.
The 24" is not going to be as stiff as the 20", so a FF tube will be more important than on, say, a shorter bull bbl. I don't think the 24" HBAR is going to be inherently any more accurate than an equally good 20" though, unless you go to a bull profile, and even then velocity is probably all the diff you will be able to measure.
Sorry for rambling...
