First off, this question has been answered thoroughly so if you want a lot of opinion on this, do a quick search.
The consensus seems to be Fusion. If you're only going to be running it for a while and don't need some of the fancier features of either Fusion or Parallels, try VirtualBox (it's free).
There IS a lot of information out there but some of it is dated and it still doesn't seem that clear cut to me, especially given the recent article on
speed.
To summarize what I've read, it seems parallels seems to get better marks on speed from this article and some customers. However, parallels has a history of problems with stability and support. however, it isn't clear to me if this is still true with version 4.0 of parallels.
also fusion seems to get the nod on business type situations. but actually most posts just say "choose fusion", and talk more about negatives of parallels (or actually the history of negatives) rather than what makes fusion so great.
what I really haven't seen is a comparison of features that one can do over the other... if there is any real difference.
personally, i was about to pull the trigger on parallels after that article on speed and finding it cheap on amazon, but having second thoughts. any updated info out there would be greatly appreciated.