Depends on the music or whatever you will mix. If you like deep bass, those samsons may make you over compensate on bass frequencies (not used the mediaone4a, but speaking from experience). Headphones are ok, but I prefer open backs for mixing; the M50s are closed back. That said, I've read that the M40 (precursor to the M50) was great for mixing so, try out the M50s first if they are what you want.
At your budget, you would be hard pressed to get any sizable monitors with large enough woofers.