For business apps, Windows Forms is very mature. It provides a gentle path from auto-generating a lot for you into allowing fine-grained control and rolling your own. There are tons of high-quality third party controls and a large body of examples, docs, etc out there. It's hard to run into a problem that someone else hasn't solved. I highly recommend acquiring some background Win32 knowledge (e.g. Petzold) as the WinForms framework lives on top of it.
I have no WPF experience, but from the sample apps I've seen it looks like a good choice for apps whose interfaces would benefit from more graphical metaphors. So if you're doing a banking app, probably not worth the extra design overhead. But if you're doing, say, a warehouse management app it could be improved by dropping pretty boxes into pretty bins.
@StephenCox: wrong answer to the wrong question. OP is asking about desktop client apps, and moreover, WPF != Silverlight.