It isn't yet much of a comparison, but http://www.haskell.org/haskellwiki/Web/Comparison_of_Happsta... has a some info; probably the only thing unique to that page is a list of some sites using each.

I'm using Yesod now simply because it seemed to have the most active community and I don't mind a lot of choices being made for me, since I'm also learning Haskell at the same time. Maybe I'll feel differently later.