I have both an Express and a Mark IV Widebody combo. They are different beasts. I'll assume you know about your Express, so I'll talk about my experience with the Mark.
You have 3 channels, Rhy1, Rhy2, and Lead. Rhy1 and Rhy2 share some of the tone controls, and this makes dialing in each of these channels really really difficult. You have to compromise a little, because once you get one set just right, the other will then be thrown off.
There is one global reverb control.
There are two volume settings, off and LOUD! Seriously, this is a loud mother-you-know-what of an amp. I have never had the master output higher than 3, and it usually stays between 1 and 2.
The graphic EQ is really cool, but it's easy to go overboard. I hate the scooped metal sound, but it is kinda fun to have that option available when you're just screwing around.
I say if you are looking for a new amp, keep your eyes open for a used Mark V. I don't know your age or physical condition, but I'm 27 and in pretty good shape. Lifting the Mark IV combo is difficult. It's almost impossible to get it in certain areas without help when it's in its case. The small difference in cost between the combo and a head/cab is TOTALLY worth it for Mesa's large amps. If it will be sitting in one room, combo all the way. If it will be gigged, or even just hauled around from time to time, get a head/cab.
Anyway, that's just my opinion of the amp. It's a killer amp, and there are amazing tones to be found in the Lead channel. There are also lots and lots of options to tweak the tone with pentode/triode, push/pull knobs, etc, and the tone controls are VERY sensitive!
Good luck no matter what your decision. Oh, one last thing, maybe poke around the Mark IV thread on the forum and see what others are talking about.