I have both. Some non-audio considerations:
1. in my opinion, the build quality on the M50s is overall better. My Beats have already been back to Monster for a warranty replacement (to be fair, the process was painless)
2. the M50s give you more flexibility in playing with various amp combinations, as they have no active electronics or noise cancelling circuits in the signal path
3. having to remember to carry spare batteries and to turn the Beats on/off is a minor pain
4. in my opinion, the comfort level for extended wear for both is about equal
The bottom line for me - unless I was trying to blend in on a college campus, I would pick the M50s
on the other hand - my kids at college report that Beats wearers are more often a crime target - because Beats = $ and because Beats are more desirable to steal.