I went from a DC-5 to a Mark IVa and I prefer the Mark IV.
The distortions sound very similar to my ear, and so did the cleans. I haven't found my perfect clean in either amp (the perfect clean tone can only come from a Lonestar), but I've found some **** good settings on both. Rhythm 2 on the Mark IV is kinda funny sounding, but whatever I did recently I've found a very nice, almost Plexi-like crunch from this channel, and it sounds fantastic with my Strat. I prefer the reverb on the Mark IV to the DC-5, to the point where I had my amp tech put in a reverb footswitch jack.
The biggest immediate difference I've noticed between the two is that the Mark IV is a good bit more compressed and, perhaps it's my preamp tubes, has more balls. The Mark IV is far more versatile as well - I can easily dial in tones from Andy Timmons to John Petrucci (think Once in a Livetime) to early Metallica to John Mayer and even Vai-esque tones. I have the Mark IVa combo with a C90 in it, and I've hated that speaker in both the Mark IV and DC-5 combos. It's one of the stiffest, least satisfying speakers I've ever heard. That being said, I normally run my Mark IV through a VHT 412 cab with V30s in it. Yum.
The only thing the Mark IV could possibly be missing is a Solo level. And although my Stiletto Ace (my first Boogie) had the Solo option, I don't miss it one bit.
All said, the Mark IV fits my ear and my personality better, since I'm a tweaker.