Depending upon your hardware, performance may be an issue, however, I'd say the main consideration is what you want to achieve. If you're main interest is in seeing how Windows 8 works, a VM should suffice. If, on the other hand, you want to see whether Windows 8 will work correctly with all of your existing hardware and drivers, you need to run the OS on your 'real' system, otherwise Windows 8 will only see the visualised environment.