We dealt with this question about 9 months ago. We decided to go with WPF and so far we're happy with the decision. Yes, there is a learning curve. It's fairly considerable especially coming from WinForms where you have so much to unlearn. I also recommend you have access to a designer otherwise your application will probably look a bit shabby. Also be prepared for some WPF gotchas that will have you spending hours scratching your head saying 'why was this so hard'.
But WPF is a step ahead. The data binding, templating and complete control of how you want your windows to look makes you think that this how WinForms should've been originally.
Oh yes, and be prepared to shell out a couple of dollars for some missing controls. There are a couple of things missing like a Date picker and having checkboxes on tree controls (you can actually template this out, but it's not as simple as winforms in that regard). 3.5 SP1 includes a grid control now, thankfully.
I'm sure I'm missing a bunch more, but that's what I can come up with off the top of my head.
Good luck!
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