Hands-down, C# was a much better language. Visual Basic (the language) always left me feeling dirty compared to C#. Both languages have similar learning curves for a novice; but C# includes industrial strength features that allow code written by a novice to be refactored instead of rewritten when the need comes along.
Both languages have a similar way for designing a Windows Forms UI, and the API for working with the UI from code was generally similar. Once C# came around, there really wasn't a reason for using Visual Basic 6 for a quick-and-dirty UI because you could do the same thing in C# in the same amount of time. (So why waste your time with Visual Basic at that point?)
I personally think the "VB.Net" situation is what really killed Visual Basic. There really was no reason to force everyone to rewrite their projects, because VB.Net didn't "add" anything. Instead, VB.Net turned into a smell that someone wasn't savvy enough to use "real" C#.
I personally don't know when I'd pick Visual Basic 6 over C# for a Windows application; but I have a pretty serious development background. I also noticed that novice developers tended to come up to speed with C# rather quickly, so I'm not sure if the more casual nature of Visual Basic 6 really helped anyone.