There is a similar question at http://stackoverflow.com/questions/202079/wpf-versus-winforms which has some good information but in my personal experience I see no reason to use WPF over Winforms. With WPF initially there was lots of talk about it's multi-threaded functionality but in use I see no benefits.
I have two applications that do the same thing, one in WPF and on in Winforms. The Winforms application blows the WPF application away in terms of performance (by a factor of 10) and looks just as nice.
Granted I am more proficient in Winforms applications than WPF.