The Audio-Technica M50s are of much, much better build quality (real leather); the beats are cheap, flimsy plastic.
The Audio-Technica M50s don't have silly electronic chips in the signal path, requiring you to replace batteries in your headphones, this is ridiculous (I have never had to turn a pair of headphones on).
The M50s have superior bass quality, it is much clearer and much more detailed. The mids and highs are both very accurately represented and with great clarity and detail.
The Beats have a larger bass quantity that is muddy and drowns out the mids and highs. It is not difficult to make loud bass, but it is difficult to reproduce bass accurately.
If you want more bass, you can always EQ, as the M50s respond very well to EQing. Maybe get a nice headphone amp that includes a bass boost.
Here's a review of the Audio-Technica M50s http://www.youtube.com/watch?v=oPfPFWefdVI&feature=related.
Here's a comparison of the M50s with the top-of-the-line $400 Beats Pros http://www.youtube.com/watch?v=et_PWifUd1w
You final decision is essentially this... Do you want:
A.) A headphone with great bass, great mids, great highs, accurate, exceptional sound quality. Great separation, detail, clarity. Good with rock, hip-hop, electronic music, rap, etc. (accurate reproduction of source material) *Extra points for rock-solid build quality* headphones for sound quality
B.) A headphone you can show off to people (because they're beats, not because of sound quality), that has muddy, loud bass, and mids/highs that are of substantially lower quality, and requires batteries to play anything. headphones for show
If you want A, get the Audio-Technica ATH-M50S, if you want B, please do not buy anything other than beats.
P.S. - You really don't believe Dr. Dre wears beats for actual music production, do you?
what headphone is he using in this pic? it looks nice