WPF is a totally a different and a new way to look in to the UI architecting and implementation. The cool concept of collaborative development by a designer and C# developer is a biggest advantage(XAML markup actually gives this ability).
When you develop a control/UI it will be in a 'lookless' manner and Designer/Integrator can take that same project(XAML) and style it for a greater look and feel. So in short WPF or Silverlight is a paradign shift in the way we do winforms development.
So .NET3.5 UI design can be done in two ways. 1) Winforms way 2) WPF-XAML way. I think for a futuristic and modern UI you definitely need WPF than winforms.