As someone who was described as having "an interesting career path" by his manager due to simultaneously developing Windows device drivers and GUI code, I say learn C#. The availability of device driver writers is shrinking rapidly, as my search for one in the last year proves. However, it's shrinking because the market is shrinking. Might be more accurate to say that the market has already tanked. At the same time,…
Disclaimer: I've worked with Windows device drivers, and there's no way I can see you could use C# there. Maybe in UMDF, but you certainly can't do anything timing sensitive in user mode. And drivers are often timing sensitive.