GugliermoI tried both last week. M220 is a decent studio headphone, that means it is neutral and flat. HE 350 is a V sharp headphone.
Back to the sound, Bass of HE350 is more stronger than M220, M220 lack sub-bass. But I prefer the high frequency of M220 because treble of HE350 is too sharp for me. for mid range, I think both of them is similar.
Over all, M220 is a great headphone for this price. About HE350, I don't have too much complain except the treble.
PS: I am selling my M220 on Ebay (Australia Only, Sorry for this AD.):
http://www.ebay.com.au/itm/272686117579?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649