I've tried a few MKVs, own a Mk IV, a VH4, a Hagen, and owned a Herbert. The MkIV is more the core Petrucci sound than a MkV is or at least that is what I garnered with my time with both.
You can get a really tight rhythm sound with the VH4 but it won't excel at recreating the Petrucci sound (it has way more girth and lows and sounds like a mastered/finished guitar track straight from the cab), and you can get gobs of liquidy gain in channel 4 to emulate his Road King/Train of Thought-era stuff. The other Diezels have a very different feel from that, they are a bit more saturated and not as dry.
What I have noticed is that Marks are very dynamic amplifiers, they will make you pay attention to your picking hand. The VH4 has a very similar characteristic, there isn't a whole lot hiding your mistakes there so they are similar in that respect. Part of the Petrucci thing is balancing that dynamic, precise amp style with pickups that are on the compressed side. Add a Mk IV to that list as well, they are fantastic amps that can be had for relatively cheap now and their lead channel does it better than any of the lead modes on the Mark V for me (YMMV big time on this last statement).