I prefer sound signature of the M90. The bass on both goes down deep, but the M90 manages to also be punchy. The highs of the Mikros are better...a little less strident. The mids are about the same. The presentation as a whole (bass, mids and highs) is just better put together with the Mikros. Both give you great isolation. The K550 has the advantage of being a full-size headphone - the sound stage is much wider and the comfort is unmatched.
Both are great cans...and both are extremely affordable.
u can do this mod if u get the k550...takes the stridency away from the highs.