They're both absolutely amazing at what they do, I have to say that Usain Bolt is more amazing though, he broke 2 world records this olympics that I thought wouldn't be broken for years, guess I was wrong. I was definitely expecting phelps to break some serious records (not 8 golds, nobody could have predicted that.
Usain Bolt
100m (9.69)
200m (19.30)
too bad he doesn't want to do the 400m though
iaaf.org - Bolt - 100, 200 . . . 400m?
sort of surprising, but if I were him then I'd be trying to pack those records in a bit before moving on.