I know C/C++ but am trying to learn C# as creating GUI based application is much easier than in C++.

I've been leveraging the options of which GUI tool I should learn. Is it best to learn Winform, WPF, or other? I recently tried out the Windows 8 preview to check compatibility with some of my old programs and it seems there is going to be a much larger emphasis on xaml so I'm leaning towards WPF. Any recommendations?

I'd go for WPF too. My knowledge is pretty basic at this stage, I haven't devoted the time to learn it but the ability to really make the UI look how you want is a big plus. Winforms are pretty basic in comparsion.

If you are concentrating only on the UI, then I would prefer you to go to WPF, it can give you rich GUI. have a look at expression blends also.

WPF is the future of GUI building in Windows. Go for WPF. MS seems trying to get people to move to WPF and there has been no new development in Winform.

It is still good to know Winform though, which shouldn't be that difficult if you come from C/C++ background and are familiar with MFC or Win32 API. Winform is what MFC should have been, with well-designed APIs.

WPF is indeed the way to go for UI programming in windows.
Keep in mind that the learning curve is high, that WPF is more or less about wistles and bells.