I have both & love 'em both.
Either one will serve you well.
If you get a '66, you must reinforce the tab on the loading gate as it acts as the cartridge stop.
A blob of JB Weld will get the job done.
A '66 has no lever safety and can be fired out-of-battery.
A '73 has a lever safety that can be made ultra-light & still be functional
I assume yer shooting black powder.
44-40 is the caliber to get or plan on doing a lot of cleaning.
44-40 effectively seals the case mouth to the chamber and very little fouling gets back into the action.
I have carbines, short rifles and rifles.
Even had a 30" '73 once, but it was in 45 Colt & I never shot it, so I sold it.
I like all the barrel lengths.
One would think that a carbine is much "faster" than a 24" barreled gun, but it is not so.
Some of the fastest shooters I know shoot a 24" rifle.
Just be sure to stay away from Chaparral guns.
The low price looks attractive, but they are a mess inside.
I recommend an action job by a competent cowboy smith.
I have had mine short stroked & like 'em that way as it helps keep the front sight on target by minimizing movement when levering.
I also recommend a lever wrap as it helps keep yer fingers in the lever during the stage.
The other thing to consider is your "persona" if yer into that sort of thing.
My Henry rifle or '66 seem to go better with my cap guns.
Too much fun
--Dawg