Back to Mangini, this is how I see it... I might be wrong but that's how I feel it...
Mangini is incredible! Maybe, in technical terms, the best drummer in the world. But Portnoy sounds more musical and melodic to me. While Mangini doubles the guitar and the other instruments and makes some moves in the speed of light, Portnoy thing (in the good days) was to add melodic phrases to the songs.
Take, for example, Under a Glass Moon (0:26-0:40), Metropolis pt. 1 (0:40-1:00), 6:00 (0:00-0:10), Innocence Faded (0:22-0:50), The Dark Eternal Night (0:09-0:25) and so on...
Drum phrases are there adding to the melodic main line; lines over lines given by the other instruments!
MM does not do that. He's got a different approach. MP was creating and adding in spite of the other instruments. MM, on the other hand, seems to follow the other instruments. And he is great doing it!
Sometimes I picture him looking at Petrucci and delivering a challenge: "Let's see who is faster?"
I somehow agree with this, but I would bring in two qualifications.
First, I think you have to qualify what "melodic" means. I guess you mean that MP creates a rhythmic pattern that is distinct from the other instruments, and I agree, MP does that a lot more than MM. But I would argue that MM is more melodic, in that I define playing a melody as a combination of playing with both rhythm and pitch in mind. MP does rhythmic patterns, but not really melodic patterns. MM, on the other hand, is very melodic in that he is conscious not just with the rhythm, but also with the pitch and the notes. For example, his intro to Breaking All Illusions very melodically sings using the cannon drums to approximate the intro riff of the song. Another example is his drum roll in the transition to the first stanza of The Enemy Inside where he played the toms melodically to approximate the melody of the guitar riff. There is also this part in TLG, the drum roll in 2:10-2:12 where you can distinctly hear drum rolls that are melodically singing.
His cymbals work is also very melodic, shifting hihats and rides depending on whether the section of the song goes up or down the scale. Thankfully it's very easy to hear because of the stereo set-up of the recording of his drums, so you can hear when he is hitting the left or right side of his drum kit, which could guide you to whether he is using cymbals with higher or lower frequencies. One of my favorite melodic cymbal playing by MM is his cymbal work in the instrumental section of The Bigger Picture. Listen to how he composed the cymbals, especially in contrasting the second half of the instrumental when the guitar harmonies kick in.
So I would say that MM is actually more melodic in his playing than MP. At least in the traditional musical sense that melody means playing both rhythm and pitch.
Second qualification is about MM "following" the other instruments. "Following" implies a very passive participation. I would say that MM's drum patterns are generally more in sync with what the other instruments are playing compared to MP, but it is not necessarily following. How can we be sure if it is not the other instruments that followed MM's drum patterns in some parts of the songs? I have said this in other threads, I suspect there are sections of their songs where MM might have taken the lead and the others followed. One is the "Mothers for their children..." part in Illumination Theory. The music in that section is so attuned to how MM would divide a rhythmic section based on his Rhythm Knowledge system that it's hard to imagine that he is not the one who lead in that section (based on his YT vid, it's 3-3-4-2, 3-3-4-3). Another part is the instrumental section of Surrender to Reason. MM is the one anchoring that instrumental, so I don't think he's the one who "followed" the other instruments in that section. I would say that MM plays more in sync with the other members, but not necessarily that he "follows" the other instruments.