Parallels actually performs slightly better for me, and the Windows UI seems slightly more responsive in Parallels than in Fusion.
Comparison of Parallels Desktop 3.0 and VMWare Fusion*
Both have:
-Fast CPU virtualization
-A feature for Windows OS integration
-CPU usage of ~5-10% when idle
-Shared folders in Windows
-Mouse synchronization
-$80 retail price
-Boot Camp support
-USB 2.0 support
-Limited DirectX 8.1 support
-Online support forums
-A free fully functional trial available
Fusion
Pros:
-dual core support
-64-bit guest OS support
-compatibility with other VMWare products
-better support
Cons:
-very few updates, in fact none yet since 1.0 (this is bad because if you start having a serious problem because of a bug in Fusion, you are stuck waiting for the next update)
-very few features (many features that are in Workstation are missing, and the feature set does not match up to Parallels)
-no guarantee of any new features without a paid upgrade
-no information about if certain features will be added, and when to expect them to be added
-slow graphics, even with experimental 3D acceleration
Parallels
Pros:
-Faster graphics (2D and 3D)
-More configurable
-More features (SmartSelect, mount Windows disks on desktop when the VM is running or when it is off, OpenGL support, etc.)
-Frequent feature updates
-Some idea of the future roadmap is given (Parallels says that future updates to Parallels 3 will add Aero support in Vista, dual core support)
Cons:
-Updates are so frequent that bugs are often introduced (which you may or may not encounter or notice)
-Poor phone/email support
Overall I don't recommend Fusion because the usability for the average user is actually worse despite VMWare's attempt to make the interface simple. For integration between OS X and Windows that just simply works, Parallels is much better for the average user. On the other hand, if you have a higher computer proficiency level, Parallels provides configuration options and tools for you to do what you want. Fusion may have the configuration options you want hidden away somewhere, but good luck finding them! Even things in Fusion that should be simple (such as adding an existing virtual disk file to a VM) require opening up the VM configuration in a text editor and making the changes manually.
Some people recommend VMWare Fusion for stability, but the latest version of Parallels Desktop does not appear to have any known stability issues. You really should take advantage of the free trials though and use your experience with the trial versions of Parallels and Fusion to make a decision.
*Comparisons are based off the latest publicly available versions of Parallels Desktop and VMWare Fusion as of 9/15/07