I have both of these headphones:
1. M-80s- not even close on this one.
2. M50 has wider soundstage, while M-80 is deeper
3. M50s are a little more comfortable, however the M-80s have improved significantly over time. They are quite amenable to twisting which allows you to adjust them so that they sit flat on your ears.
4. They are fairly even in the bass department. M50s goes a little deeper, while the M-80s have more mid-bass thump to them.