Go for the 8.5..I really like Sperrys. I feel they are incredibly comfortable :) I have about 4 pairs and they fit amazing :)
I had the same problem as you picking my first pair.. I've got really fat/wide feet. So for my first purchase, I got a 10.5.. At first it fit nicely, snug, comfy.. no pain whatsoever... but then it ended up becoming a bit loose (had them for about 10 months now). I can feel the back of my heel banging into the back wall of the shoe (not that it hurts mind you) and sometimes feels like it can come off. My next couple of purchases were 10's. Fit a bit tight for the 1st-3rd wear. But now I have no problems... comfortable, if not more than my first pair.
I suggest you go for the 8.5 it MAY seem a little uncomfortable for I'd say, at least 3 days wear.. but it'll be fine after that :)
