I guess it really depends on what you want to do.
Basically, the difference is this. Parallels is what they call a virtual machine, which is when you have a completely isolated instance of an operating system running inside of another operating system. In this case, Windows would be running inside of OSX. The advantage to this is you can still get at all of the programs you run in OSX while you're running Windows. The disadvantage is it's super system resource intensive because you're running two operating systems at the same time... which means it can be super super slow depending on what other programs you have running.
Bootcamp is what they call a duel boot partition, which means you have two operating systems on your machine (OSX and Windows) and you use them one at a time. Disadvantages being if you're in Windows and you need to use a program in OSX, you have to logout of Windows and boot into OSX, but the advantage is it's less system resource intensive.
I've only ever used Parallels and I was using it for web development work and I needed Windows to test sites in Internet Explorer, so even though it was slow, Parallels worked fine for my purposes... I'm just not sure I'd want to to use it for much else personally.
