I have the
85/1.8 and I've reviewed it here:
http://www.bobatkins.com/photography/reviews/canon_ef_85mm_f18_review.htmlI'd say that the 85/1.8 is probably the better choice for several reasons.
First, on a crop sensor camera the 85/1.8 equates to a 135mm on full frame - the longest "classical" choice for a portrait lens. The FOV of the 100/2 equals that of a 160mm lens on full frame, which is still fine but I'd chose the shorter focal length.
Second the 85/1.8 is a little sharper wide open based on numerous internet reports and tests. Both are sharp lenses, so this isn't a big issue, but if you add it to #1 and you also add in the fact that the 85/1.8 is a little cheaper and 1/3 stop faster, all those factors would (and did!) push me towards the 85/1.8.