I'm a MP fan, but MM is more of a technical player, that much is clear, but technical ability is nit what makes a good musician, it's only part.
Anyway I'm not going to make this an MP VS MM thread, so I'm not going into depth
Yes, but how good the musician is might be the subjective side of the arguement. MM is ahead of Portnoy in the objective side, which is being more technical (it's the only one you can take note of on paper through sheet music and so on, I think).
Perhaps Portnoy has done more projects and that shows his abilities to blend in into many sorts of music and still bearing a great chemistry which such a broad range of musicians, that could actually be a sign of who is the "better" musician, but we might never know if Mangini's will is not to participate in lots of projects so that he can properly say "I can play pretty much everything and with anyone, too" (like MP could do if he wanted to because of how many bands he is/has been in successfully). At the same time, I don't think it is MM's will to do that, he has truly found his place in DT and he might be doing a better job than MP did in the band in the musical sense, or at least so at the moment (of course, comparing how they work with MM in the band and how they worked back in BC&SL which was where we heard MP last time in DT).
MM has demonstrated being better in some ways, but he hasn't been able to show that he is the sort of musician who can play with anyone/in any genre (other than being a Berklee teacher, considering he must have gone through teaching all sorts of music to students). MP has participated in so much stuff that has been really successful in the musical scene, just like TWD, TA or FC, that at least I can tell that MP has demonstrated he can play in more bands and be good at it. That might come from MM not having shown it, even though of his ability to do so. I guess that's the part that can't be demonstrated and there is the point whether MM or MP is better. MP has worked successfully with more musicians than MM (as a band member), while MM has demonstrated being more technical than MP through different ways.
I respect and see what you're saying here. But I'd say that it's arguably true that MM has demonstrated as much if not more versatility as MP. Sure he hasn't been involved in as many projects - but the musicians and bands that MM has played with shows in my opinion a broader spectrum than MP. Weren't Annihilator some sort of Death Metal band? Not to mention Steve Via and Mike Keneally and most notably Extreme.
As said, yes, Annihilator is a thrash metal band, and that heavy element is still retained not only in certain tracks in which MM has been the drummer for DT on studio versions, but it's there in older songs and it is (thrash as a genre) a huge influence on DT, or at least so for some thrash metal bands. As for Steve Vai, Mike Keneally and Extreme: sure, the three cover a rather wide array of genres and are demanding in their own ways. In his case, MP has done DT, LTE, Neal Morse, Transatlantic, OSI and John Arch - only to mention the progressive rock/metal bands he has worked in, which all require a huge amount of versatility. Outside of that, A7X, Adrenaline Mob, The Winery Dogs, Flying Colors most of all the mentioned ones quite successfully and for rather long periods of time, and his really-hard-to-keep-track-of list of bands he has played with live.
I don't mean that MP is more versatile than MM because he has played in more bands because, as said, it hasn't been MM's will to work in that many projects either. What I mean is that, if we were to look for facts, MP has recorded successfully with more artists and shown his abilities properly in the related genres more than MM, which I believe should be taken into consideration when saying which drummer is better. At the same time, I don't mean that MP is better than MM either (I prefer MM a lot more, to be honest), and MM has an amazing background which could support at showing superiority as a drummer over MP.
But, in the end, there's also a huge chunk of subjectivity, not only at telling who is the better drummer because of any listener's taste, but also in everything I said there, which are a couple of the standards I guide myself by when telling who the better musician is.