XP hands down. More stability, more compatibility, in my opinion faster since it doesn't use as much overhead as Vista does.
You said you had 4g of ram, keep in mind XP 32bit will only see 2g, while XP 64bit (not totally compatible with most software) will see all 4g. No biggie since your running in Virtual Mode.
and technically you are not allowed [per the Microsoft License] to run Vista under Virtualization anyways.
Actually you can, unless they lifted the virtualization license running Vista will cost you more since you'll need the Business versions rather than the Home/Personal versions as MS decided they didn't want people running the cheaper version in a virtual machine. The license issue was only restricting home/personal editions, business and higher can be ran in virtualization.
As far as Parallels/Fusion, eh, they bost cost the same and since your only doing it for a few small apps, I don't think one or the other will matter. Lots of people here use virtual windows for more than just one or two apps so they/we tend to be biased to a particular software *cough*fusion*cough* but I think you'll be fine.
Recommendation: XP