Help me decide.

[Russ]

The size of the head, or being in a combo will have nothing to do with the sound. The short chassis heads are just the same thing that's in a combo, but some people want their heads to match the size of their speaker cabinets, so that's why there is a variety of sizes.
The halfback cabs are phenominal, even with the "dreaded" vintage 50's. I've heard excellent reviews of the halfback 2X12's, and I think some of those were ported as well.
Note, the halfbacks can still be sealed off at the top with a piece of plywood if the back cover is not available.
Is there a local music store that has a Mark IV you can try? If so, see if they will let you crank it up. I'd jot down some settings before you go if I were you, they are a bit intimidating at first to dial in.

The wide combo tends to have a little more bottom than the short combo. Otherwise the wide chassis just gives you more front controls where the short chassis has a couple more switches on the back. Short chassis amps can be rackmounted while wide cannot. toaj has noted that the short will fit in a rack even if in its head shell. Mark Series halfback 2x12 cabs are ported in the bottom.