I've had no issues with WPF in ArcGIS whatsoever. The question of chosing one of these technologies over the other is the same as in any other environment, be it ArcGIS or not.
There are things to consider, though. If you want to use WPF as replacement for your forms, then you are definitely OK. If you'd like to use WPF in scenarios like e.g. docking windows, you need to be more careful since WPF/Win32 interoperablity can have subtle issues and performance impact. In my experience though, these arise rarely, and in very corner-case situations.
Another thing to consider is whether your applications run in environments like Citrix or are very often accessed via remote desktop. In those cases, the WPF drawing pipeline can easily slow things down, especially (but not only) when you make use of advanced effects.