He must be going deaf if he honestly said the MkV sounded better than the 2c+ (or the most probable scenario, got paid to say so). I've heard numerous clips and played that amp so many times, yet it still feels and reacts like a 2c+ modeler.
If you like the Mark series tone, I'd recommend you either find a used 2c+ or IV.
Well Mesa actually doesn't pay anyone to endorse its products. In fact, they don't even give out free products, so even people like John Petrucci have to pay for their amp. So I don't think it's a conflict of interest. Anyway, it could have been something as simple as the IIC+ was in need of repair since it's so old and he just found it easier to use the Mark V.
Have you played the Mark V and the IIC+?
I think that the whole "Mesa doesn't pay/offer discounts to endorsees" statement is true for the majority of players, but I guarantee that's different for heavy-hitters such as JP (and kept under wraps, of course). I'm pretty sure that they know he influences gear-purchasing habits in a major way (check out his forum if you don't believe me). I wouldn't be surprised if they ride the gravy train to the point of designing a "MV Lite" which just has a couple channels / features for a lower price point.
I doubt his 2C+'s ever need repair. Mike B. (the man that did the original C+ thing) still works at Mesa, and he routinely services those amps. He told me that there are a few people that own more than a few C+'s and send them in for check-ups about yearly (I wouldn't be surprised if JP is one).
Like I said before, I've played the Mark V tons of times. One of my friends is a HUGE gear whore and goes through amps like no one's business. He had a MV and we tweaked with it for a good month or so (even played a show with it). It's super versatile and very convenient when it comes to features, but it's one of those "jack of all trades but master of none" scenarios. It'll get you 80% there or so, but there are better amps for each of the modes it attempts to emulate.
As for the 2c+, I've had one for the last ten years or so. It was serviced this year by Mike B. I've gigged with it tons, and play it on a daily basis.
In my opinion, there are "better" amps as far as pure tone is concerned. If you want features and versatility (if you like a wide array of tones for performances, IE cover bands) then the V is great. In a live setting, there isn't a noticeable difference between awesome tone (80%) and perfect tone (99%).
Another option for acquiring an amp is to look for a used 2C and have it sent in to Mike to be upgraded to C+ (I think it's around $500 or so). He still offers that, as well as upgrading to Simul-Class, EQ, or Reverb.