I'm in a similar boat as OP... trying to setup a Win7 VM for my girlfriend on the MBA she's getting for Christmas, but haven't been able to pick a clear winner between VMWare 3 and Paralles 5 yet. I've run some initial experiments on a leopard mac pro:
VMWare: I really like the Fusion menu. Takes less real-estate than Parallel's combined menubar icon and Windows apps folder. Also combines both VM functions (preferences, suspend, and exiting unity) and Windows apps into a single menu, whereas Parallels I have to right-click for apps and left-click for a stripped down VM menu: Devices, Exit Crystal, and Quit. How hard would it have been to provide access to VM preferences, or other view modes, without requiring me to exit Crystal mode?? Took me forever to figure out that to suspend I needed to Quit, and then answer the pop-up dialog too. Also can't understand why the most common function (Start Menu) is assigned to the least-used mouse button...
Parallels: Like others have said, graphics is more stable. My gf runs several apps that render their own UI's, and these have serious problems in Fusion (transparent overlays in Fusion appears as huge rectangles that can't be clicked thru, and dragging is a stuttering mess). Normal apps like IE or Word seem fine in both though. Hate the parallels icon overlays (the two orange bars), wish there was an option to turn it off, makes navigating through the Windows Applications folder an ugly mess.
I'm really hoping that Fusion's rendering problems go away on Snow Leopard, or they issue a fix later. If so, I think it would definitely be the VM of choice for an end user who isn't very familiar with how VM's work, and just want to be able to run some simple Windows apps now and then, seamlessly on their mac. Curious what others think...