I have the M30's, and the only Beat's that I've ever heard are the Solos.... and I would rate those two very closely.
The build quality isn't great on the Solos, and while the M30's do feel of better quality, they tend to creek while I'm using them.
The comfort is similar; the M30's are technically over ear, but have very small ear cups.
Bass in both is muddy, but the M30's have much less sub bass. Hip hop sounds better on the Solos, most of everything else I'd prefer on the M30s.
I haven't listened to any other Beats, but from what I've read they are much better than the Solos. Even though most people on here bash the Beats lineup, I don't think a $40 headphone in the M30s can really compare to those. The M30's do sound like a $40 headphone. I like them, and they have good value, but I doubt they would come close to the quality of the better offerings by Beats.
What is your budget? What kind of music do you listen to?