Boot Camp is going to offer "bare metal" performance. There is no virutualization so the performance is the same as a PC with identical specifications. Parallels runs on top of OS X and therefore you are balancing system resources between the two operating systems. A neat option in Parallels is that you can install Windows via Boot Camp but still use Parallels when in OS X. When you need full "bare metal" performance like for Photoshop, gaming, CAD, etc. you can boot fully into Windows.