Vista is basically a test run. windows 7 preforms better and is much better optimized for ram,cpu,gpu ect usage. I would say windows 7. But on either one I would disable superfetch.
Windows 7 is the obvious choice, but why on earth would you disable superfetch? It caches the programs you use most in your ram, so they open pretty much instantly, instead of waiting for them to be read off the hard drive.