It clearly depends on the situation. If you need to fully run Windows iThink Boot Camp is the only way. For my situation i had a specific program that did not need Direct X 10 or anything. So virtualization software fit my needs and then it was just Parallels because of promotional pricing..
It all depends of what you're doing. If you need to play games or other "intense" graphical programs than definitely go for Bootcamp, as it runs completely native, and you'll be using the computer to its fullest potential. However, if you're just using basic Windows applications, and don't want to switch between the two operating systems, get Parallels.