Both are great choices, but have different strengths. I have the IE8 and have only listened to the W3 for a short period of time, but here are some thoughts:
Both have great dynamics, but the IE8 requires a lot of burn in, say 400+ hours, maybe 1000 to be on the safe side.
IE8: wider stage, more laid back forgiving presentation, better treble extension (not that the W3 sounded lacking or dark), less mid-bass hump (after burn in)
W3: a little more detail and instrument separation, more forward presentation, nice 3D stage
I do think the IE8 is great with metal & R&B.
EDIT: Oh, and both have reported fit issues. From what I have read it seems like the IE8s have less isolation and the W3's have more overall it issues. Basically, both take work to get a good fit for some people, more than most other IEMs.