I'm still going with a Galaxy Nexus. I am fed up with skins and I see rooting and custom ROMs as something that you do once your warranty has expired, or when the phone becomes unusably slow/short of memory.
Jelly Bean will give it a real boost. I doubt the GSiii will get that until next year.
I had a Galaxy Nexus for temporarily and loved it, and the user interface design is very pure and consistent somehow.
The advantages of the GSiii will be raw speed (would be interesting to see the comparison after the GN gets Jellybean, but the GN is pretty quick anyway), plus a few random bits and pieces like Kies, built in Swype, and that feature where the screen doesn't blank while you're looking at it. I can live without these.