We use Snap, but frankly if you know one, you can learn the other in a matter of hours. One of the nice things about Haskell is that the API often speaks for itself, and for all their differences, their APIs are actually fairly similar.
Also, it may be of note that I am one of the core contributors to the Snap Framework.
Also, it may be of note that I am one of the core contributors to the Snap Framework.