The Beats are more comfortable than the M50s. Those new pads on the Beats are extremely soft and they apply the perfect pressure for my somewhat small head. The removable cables with the Beats is also a huge plus. They come with a regular cable and one with a remote and mic on it and they both seem well made.
Now in the looks department, it's no contest. The matte black Beats look incredible. The M50s look like they belong in a studio. The only down side, people may view you as a snob for wearing them in public.
For mastering music the M50s have the edge BUT I wouldn't say either has a completely neutral sound signature. The Beats are obviously "heavy handed" and they have this hiss from the active noise canceling that can't be turned off. For normal/active use you don't notice it so much, but in quiet passages you can definitely tell it's there. The M50s also have a slight bass emphasis, not nearly as much as the Beats though. Also, to my ears the M50s aren't exactly neutral up top either. They are a bit bright, and while it may bother some, I kind of like it.
For casual listening I find the Beats more pleasing out of these two headphones, but I recently bought a pair of Sony MDR-1R MK2s and I haven't been able to put them down. In my opinion, they best the Beats in every category except maybe looks.
EDIT: I don't master music, but maybe consider the Sony MDR-7506 or the headphones FiioforMeOh mentioned. I no longer own the MDR-7506s but I remember them being the most neutral out of the headphones above. Hope this helps!
Edited by ZedsDead - 7/8/14 at 8:02pm