The comparison in a nutshell:
Model '73 (includes Henry and '66)
1. Lifter mechanism lifts cartridge into alignment with the chamber, and cartridge is parallel to the bore.
This results in the bolt pushing the cartridge straight into the chamber, resulting in almost no possibility of
jamming.
2. This action is somewhat sensitive to overall cartridge length. A cartridge that is too short will potentially
allow a second cartridge to partially enter the lifter block, thus jammig the action. One that is too long,
will of course not fully enter the lifter and jam the action.
2. When worked over by a good gunsmith, this is normally the smoothest action.
3. the '73 action is not as strong as the '92, so you can't shoot "magnum" loads in it without risk of
damage.
Model '92
1. The cartridge carrier lifts the cartridge at an angle, where the bolt must catch the upper rim of the
cartridge and push it forward while the cartridge tilts down to level to enter the bore.
2. Trying to run the rifle really fast can result in stovepipe jams, as the cartridge doesn't always have time
align properly with the bore before the bolt closes.
3. The '92 is typically not a smooth as a '73 action
3. The '92 is a much stronger action than the '73
4. The '94 is a scaled up '92. Thus the '94 has the same general characteristics.
As for barrels, typically octagon barrels are found on rifle length barrels (20 to 24 inches) and round barrels are found on carbines (20 inches and less). The choice simply depends on what you like the looks of and what feels best when you handle it.
I have a 24 inch Model '73 and an original '92 carbine. I prefer shooting the '73.