If you only had the choice between the XB500 or the M50, the Audio-Technica is the better choice.
I've had the chance to try both (I actually own the M50) and the difference between them is night and day.
The Sony's are comfortable; it's like wearing a pillow between your ears. Though, I might as well describe the sound in a similar fashion... it's like having your sound muffled by those pillows. The XB500 (might as well add most of the XB#### series) produce poor sound. Hearing these headphones is like dipping your head underwater and trying to listen to the radio, it's restricted and muddy. Not to mention that these "bass enhanced headphones" don't even produce that much of a great sounding bass.
The Audio-Technica's, on the other hand, offer a very crisp and clear sound, not to mention a pretty deep sounding bass. However, nothing is overly exaggerated with these headphones, which might turn you off since you like listening to bass heavy tunes. But I can assure you, my Trance/Progressive mixes have never sounded any better.
However, if you really love your bass, I'd recommend looking for other cans. Most notably DT770 or some of the Ultrasones. Don't forget to add an amp!
Hope that helps!