Well, I don't do any critical listening, so I am in no position for a very comprehensive review. So just couple of thoughts:
1) Bass: m5 provide very good amount and extension. I would not say E500 have more bass, but E500 to me generate significantly more impact.
2) Mids: both are very good in that department.
3) Highs: no comparison here. Even though E500 are considered rolled-off, they have much more sparkle in the highs compared to m5. That actually can be bad for certain music types as E500s with rock music sometimes sound too "spashy" and annoying.
4) Clarity, soundstage, instrument separation - again my vote goes to E500, although m5 is not bad at all. E500 are just more refined headphones.
Overall, I find m5 to be warm, "fun" headphones and a step up from Vibes, while E500 are more balanced, with greater clarity and strong bass impact.
Isolation is comparable with proper seal. I use grey large sleeves for E500 and large bi-flanges for m5. Getting proper seal with m5 is much harder than with E500. INterestingly, I cannot get a proper seal with m5 using Sure grey sleeves due to their shape and I don't like foamies in general.