Personal preference. Would you rather be able to access your Windows apps while running Mac OS X, or is performance of your windows applications more important to you. For me the answer is both.
I play a bunch of games, and some of them only run on Windows. One in particular is a resource hog (Armed Assault); I don't forsee ever being able to run that game in a virtual machine. People running it on NVIDIA GeForce 8800 GTX's with 640MB+ RAM have performance issues (which is a really sad statement about the game code), so I need to be able to boot straight into windows to play. So, Boot Camp wins.
On the other hand there are some games that do run in Parallels, not to mention MS Office and a handful of other apps I want to be able to access while running Mac OS X. So, Parallels wins.
Therefore I keep both. Really it depends on what you need. Only have some office or productivity applications you want to access while running Mac OS X, then Parallels may be all you need. If you run graphically intensive applications, or you have some other situation that prevents you from doing what you want in Parallels, then Boot Camp may be what you need.
Or, if you want both, then you might want to keep both.
Jason
