For a single user installation of Windows on a Mac, Fusion and Parallels are essentially feature-indistinguishable, and its really a matter of style, interface preference, etc. There are a few differences but both are very full-featured, complete, stable and solid programs. I use Fusion (after having tried both) but both have their fans here, and both Fusion and Parallels users coexist peacefully on this forum ...
VirtualBox is free, and runs Windows fine or so it seems (I have only spent a short time with it), but does not seem to be as full-featured as either Fusion or Parallels. However, it definitely gets the job done and should be considered if your needs are limited to running just one or two programs. And as noted, you can't argue with the price.
Fusion's main advantage is its scalability and integration with enterprise-wide virtualization systems, of which they are the dominant provider. If you will be doing any development applications in your (or for other) VMs, if you will be using or trying out various Linux flavors, or would find their library of pre-built virtual machines and environments useful, Fusion is the way to go. It runs Ubuntu like a champ on my machine, for example, and I've used it to try out various Linux flavors very easily.
But for most users who are installing XP, Vista or Win7 just to run an occasional Windows program, really you can't go wrong with either choice. Both offer free 30-day trial versions - download them both, try them out and pick the one you like best.
Cheers