Depends. You have to ask yourself what you want to do with Windows! IMHO Parallels has to be update with every new version of OS X. It is known to run some more advanced games. Again IMHO if you want to run Windows games then go with a BootCamp partition and boot into windows.
Also I strongly suggest
VMWare Mac Fusion for mission critical Windows applications running in OS X. IMHO it is more stable than the competition.