I'd do virtual unless the app doesn't work that way. The need to reboot to switch to that app would annoy me to no end. I rarely reboot as it is so something that makes that more frequent is bad in my book.

The other thing to consider/try is that there is an option to run apps in compatibility mode. If that works it is way easier than actually maintaining the second OS instance, though it certainly does not always work. Just right click on the shortcut, choose the compatibility tab and see if those options work for the app.