Personally for me i managed to A/B the M50 vs the HD558 at the shop. and you can see in my signature which one i ended up buying.
Overall the M50 has very little soundstage, kind of less than grados of its price. meanwhile the HD558 has a very wide soundstage.
For games and movies the HD558 is better no doubt, since the sound is more natural, immersive and transparent, and also does vocals very well and lifelike.
The M50 can be quite prone to sibilance, while the HD558 is known to lack subbass if you do the foam mod. (but both can be fixed with eq tuning)
Imo the only benefit of the M50 is isolation and portability, and maybe a more 'fun' V shape sound if you like that kind of sound sig.
and also as stated above, the M50 doesn't hold a candle in terms of comfort. The M50 comfort is decent, but the HD558 has huge valour earpads and a very deep earcup. makes most headphones out there seem uncomfortable in comparision.
last note: the postitioning of the HD558/HD598 headphone on your head is very important in its soundstage presentation, so play around with the placement (all directions, up down front and back)