I love WinForms. It's really easy to pick up, you'll see the same concepts over and over.
I had typed up a whole rant about how much of a pain in the ass WPF is, but that's not really important. It's just really different from Windows Forms. Some things are much easier in WPF, while things that would take two clicks in Windows Forms require an afternoon of XAML editing.
WPF's data binding model took me a while to get my head around (in fact, I still only know the basics of it). Control templating is a major annoyance but the results you can achieve make it almost worth it. I feel like the XAML editor isn't nearly as fluid as it should be; I find myself messing around with arrow keys all the time. I feel the UI designer also leaves something to be desired, especially in comparison to the Windows Forms designer. I rarely, rarely use the WPF designer. I just find it easier to write the XAML.
All of that said however, things are definitely going the way of WPF, at least on the Windows side of things. I've noticed newer UI features sometimes are released only for WPF.
My two cents: Learn both. Windows Forms is really easy, at least for me. It just feels natural and completely logical to me. WPF definitely has a massive learning curve, and you'll find yourself bashing your head into your keyboard trying to do simple tasks at first, but in the end it pays off. Know the basics of WPF? Great! That means you can pretty much jump right in to Windows Phone 7 development, which is Silverlight-based. Lot of the same concepts though. Whenever I do personal projects, I decide which technology best benefits the project and go with it. That alone has helped massively with learning WPF, even if it's just stupid little apps that don't do much.
Good luck! :) And sorry for the lengthy post. I could write forever on this topic.
UPDATE:
Since I originally wrote this post, I've fallen completely in love with WPF. WinForms is dead to me (except for when I have to make small little utilities).