Wait and see
...how the features compare when (a) VMWare is on sale and (b) Parallels wants paying for the next update.
Disclaimer: I haven't tried Fusion - but I've used VMWare Workstation on PC and its an excellent product.
Things to consider:
1. Office/productivity/legacy apps
Performance of Parallels is more than good enough for this, while "Coherence" and Parallels' other efforts to integrate Windows with the Mac desktop are a killer feature - saving screen real-estate and letting me use two screens. Fusion would have to beat those.
I don't see the multi-CPU support of VMWare being a killer here - these are not CPU-intensive apps and Parallels itself is taking some advantage of multi cores.
2. Games, 3D and other demanding apps
If I want to run these then it takes 2 minutes to re-boot into native Windows and have a machine firing on all cylinders. Virtualisation will never deliver the same performance as bare metal - and the reports of 3D running under Fusion seem to be restricted to fairly old games.
3. Linux and other non-Microsoft operating systems
Parallels are clearly concentrating on the big Windows market and their Linux support is not stellar, whereas it sounds like Fusion has inherited the Linux tools from VMWare's PC product line. The lack of Linux tools make Parallels pretty hopeless for running a Linux desktop, and even for a server install you have to faff around setting up file sharing and time syncing. Having said that, there are work-arounds for most of that - including using the OSX X11 server to run the desktop to get mouse integration. Plus, while I've made extensive use of virtual linux "development" servers for web apps etc. under Windows, I'm finding them less essential under the Unix-based OSX. However, I could almost see myself using Parallels to run Windows and Fusion to run Linux.
4. Support
This is a "wait and see" issue - VMWare are used to supporting sysadmins and developers (the dominant markets for virtualisation on PC) and they've been much more conservative about how widely they distributed the beta. Lets see how they cope when hoards of non-techie Mac users try installing Windows and hooking it up to their mobile phone or running obscure bits of software. There have been a lot of whinges here about Parallels support - but I'm really glad that its not my job.
5. Price
Everybody is assuming that Fusion will be priced competitively with Parallels (i.e. cheap and cheerful). On PC, the basic VMWare Server and VMWare Player are free, and they make their money by charging serious dosh for their developer and enterprise products. I don't see that working in the Mac market (but who knows).
The real killer would be if VMWare they did a deal with Microsoft and produced a Virtual PC like bundle including an OEM Windows license. Given that they've recently been slagging each other off in public, that seems unlikely.
6. Leopard
And then, the rumours of some sort of better-than-bootcamp Windows integration in Leopard will be confirmed one way or another Real Soon Now. Personally, I doubt it (beyond the pretty well established release version of Bootcamp) - Apple can't be seen to embrace Windows too warmly.