7 is quite a bit lighter on resources as a rule while you are still able to tweak Vista quite a bit. One item is speeding up the shutdown time on XP and Vista as well as 7. Shut Down -Speed Up
I had been quadruple booting XP, Vista, and both 7 betas at one time until no longer having any need for XP, creating a backup image of Vista to make room for the 32bit RC, running Vista on the Virtual PC, and now have that replaced with the 32bit RC there! Note this while being booted into the 64bit RC host.
If you really look closely you'll find there's a big difference in various ways while it also depends on what you have running on any version as far as overall performace and having adequate memory as well.