Hmm. I don't actually own a Mark V but I think I can comment and hopefully handle some insight, at least as much as someone who plays the Mark V for 5 minutes in the store and then plays the Electra Dyne for 2 hours plus. After trying the Electra Dyne, I started fighting with my Dual Rectifier. I felt like the Electra Dyne is just so much more 'my' sound.
Step 1) DEFINITELY look up the pile of videos Don at Haggertys put up on youtube. He THOROUGHLY demos every channel on the Mark V. http://www.youtube.com/user/haggertysmusic#p/u
Step 2) Try gear.
OK, so here is a basic overview of my THOUGHTS on the amp. A Mark V is jaw droppingly awesome. It does what it does EXTREMELY well and if that is specifically what you are after, it is well worth it. The cleans are fantastic and I particularly like the high gain lead tones. I have NEVER felt a lead tone that rich, lush, and full bodied ever before. The rhythm tone is pretty rad too: very fast, clear, and articulate. There are tonnes of different sounds in the amp so if you are the world you want to be in, there are many tones you can investigate. Always a way to shape and modify things slightly. Furthermore, the 10watt power option really does turn the Mark V into a practice amp. It isn't like the 'Dyne or even less extreme like a Dual Rectifier. Both of these amps you have to 'open up' a bit for them to wake up, but the Mark V conquers basically all decibel levels period.
NOW, there are tonnes of modes and sounds, but the amp really has one consistent 'voice' at the same time. This voice is a really peculiar thing and in fact, it is the voice that got Boogie famous in the first place. It is a very harmonically rich and creamy tone, distinctive because of its clarity, richness, and evenness. There isn't that sort of raw, wild, or 'pissed off' tone you'd expect from a boosted Plexi. Rather, it is pure DJENTY goodness at the extreme and not so extreme setting on the amp. Yes, it does low gain well but that voice is always there. If you LIKE this voice, you'll be in heaven. (If I ended up with a Mark V, I'd live on crunch mode and Mark 2C+)
All things being Equal, I'd say this is Mesa Boogie's best (most flexible) amp. Sounds good from practice to gig levels, does many tones, all of them fantastic.
To explain further, if we look at the Electra Dyne, as mesa as it is, it really sounds far more crunchy and 'British' than the Mark V. The Electra Dyne flirts with modern tones (especially with high out passive pickups such as my Bare Knuckle Rebel Yells) but it primarily is about the vintage thing. When I play it, I turn the gain trim switch to 'clean' and I dime the gain. I then use my volume knobs to adjust my tones.
On this fake sort of spectrum I am drawing, the Dual Rectifier Reborn really splits the difference between the Mark V and the Electra Dyne, at least in terms of voice. As far as tone goes, thie Dual Rectifier gets the most modern with the roaring high gain settings.
The Dual Rectifier Reborn is really half British by comparison. What I mean is that the architecture / layout of the tone stack on this amp is very similar to a Marshall. The difference is that Boogie basically Americanized a JCM800, at least indirectly. (The Dual Rectifier is a Modded Soldano SLO while the Soldano SLO is a modded 5150 and the 5150 is a modded JCM800) It has modes that 'flirt' with low gain (Pushed, and Raw) in much the same way the Electra Dyne flirts with high gain. The tone is decidedly bigger than a Mark V. It is a bit slower, more sluggish, and the lead tone isn't quite the same. I find if you pour on the gain, the lead tone gets usable but it never quite has that tone that is so great that you lose spontaneous bladder control. The crunch is a different story. There is something so phat, rich, and just plain huge about the Dual Rectifier. It does not have to do the scooped nu-metal thing. In fact, a lot of the best tones can be dialed in with the tone stack set flat. A Mark V screams and a Dual Rectifier roars, if that makes sense. The Electra Dyne Purrs. LOUDLY!
An amp in the Dual Rectifier family that has a lot of options is the Road King II or if you don't have $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$s, the Road King. You can get quite a few good tones out of these amps but they aren't as flexible as the Mark V is. Of course, it is up to you but I personally think that investigating each different amp will work out in your favour, as you will be able to nail better what you want in an amp.