If you believe that MacWorld is an unbiased reviewer of software and that they have done a thorough investigation of the features, performance, etc. of VMware Fusion 2.0 vs. Parallels 4.0, then you might be interested in the fact that VMware Fusion 2.0 was awarded an MacWorld Editor's Choice - “Software offers the best choice for running other OSes on your Mac”.
I have no experience with Parallels and have only been running the latest version of VMware Fusion 2.0 for just a couple of days, but I have been running the earlier version of Fusion on my Mac and have found it very responsive, reliable, and easy to use for running Windows XP SP3, Visual Studio 2008 SP1, SQL Server 2008, and my various other developer tools on a MacBook Pro. I will say I have a 2.6 GHz, Dual Core MacBook Pro with a 7200RPM Hard Drive and 4GB of Memory. If you have a less performing Mac and/or more demanding Applications your mileage may vary.
If you don't trust MacWorld for providing unbiased and thorough reviews, you can always download a free trial for VMware Fusion and Parallels from their websites.
You can read some of my previous posts on VMware Fusion:
You can find the VMware Fusion Award here on MacWorld. The deal I mentioned the other day on VMware Fusion for only $39.99 was only a single day CyberMonday Deal, but you can still get it for only $62.99
, which is about $20 cheaper than if you buy it from VMware :)