Ive used both vmware fusion and parallels and personally I find fusion a superior product in almost every way, especially where it matters which is in general performance and stability. Im on a Mac Pro (early 2008) with 4gb ram and I run Windows XP Pro under vmware almost constantly. With only 4gb I find it a struggle to run Windows Vista or 7 and be able to do anything worthwhile with it and still run all my mac apps. But XP performance is generally excellent. Fusion has been very stable for me, no hickups at all. I run various older games under it without issue, I even run an old version of MS Virtual PC under my WinXP VM for some ultra-ancient DOS stuff which runs remarkably well considering its a VM within a VM lol. It should also be noted that Fusion works very well indeed with dual-monitors and Windows recognises the fact I have two monitors just as it would on real hardware. This combined with OSX's Spaces feature is fantastic.
As for Parallels to be honest I've never found it very impressive. It does have some nice handy features which Fusion is missing, such as some fairly comprehensive integration with the Windows and Mac side and mounting of virtual drives etc. If thats essential for you then Parallels may be worth a look. But if performance and most importantly stability is what you need, then Fusion is the way to go. And as someone else mentioned, it has excellent Linux and alternative OS support. Parallels rather oddly caused some minor performance issues on my system on the mac side when running the VM, which I never witnessed with Fusion. Also Parallels seems to get worse stability wise from release to release and as others have said, good luck getting proper support from them, they don't seem to care much. I also had issues with dual-monitor support on Parallels and WinXP.. it would use both monitors but instead of Windows recognising them as discreet monitors as it would on real hardware, it simply saw it as one big monitor and thus one big desktop spanning both my physical monitors. Odd.
Anyway thats a rather lengthy way of saying, Fusion is better
But try both, see which you prefer.