I guess I didn't adddress anything other than velocity and energy.
For hunting or other general purposes, I'd go with a 20" to maximize the .30-30 w/out having to go to a longer 24" bbl gun. If compactness is a high priority, a 16" will work well (I have an 336LTS that I really like) but IMO, the gain in effectiveness of the .44 mag (or even a .357 mag) in a 16"-18" levergun, combined with the performance loss from the .30-30 as the bbl shortens, begins to tilt the balance towards the pistol caliber leverguns if you plan on short-range work. Not having fired my 16" and 20" -ers side-by-side I can't say how much more blast you get in the 16", but I don't recall it being too obnoxious.
Preacherman, how's that Cowboy shoot? I should have bought one when I first looked at them, because Marlin is only showing them in .38/55 these days.