Hi,
Definitlty we need a .NET API in c# what ever the UI is WinForm of WPF. Personnally I am using WPF for a really long time now and it has nothing to do with WinForm, WPF is simply an other simple way to render UI, what I mean by simple is that when you catch the approach you cannot get ride of. A lot of developpers using c# are using WPF as well. WPF is a state of the art approach when you talk about databing, trigger and templating, simply try this with windForm and you will see.
But Genraly it is not a UI matter but a simple API that is capable to work with both WinForm and WPF. Personnaly I will be frustrated if I have to re code something to get it working for WPF such dependency properties, notify property etc…
Go for a logical .NET API, in c# which can be used in a DLL