Windows 7 is essentially an updated Windows Vista, so there's no reason to bother with Vista.
It's basically the same idea as Win98 vs Win95 or WinXP vs Win2K. In all cases, the newer one was just an improved version of the older one, with more features.
Side note: I started using Vista during the Beta 1 phase, and dual-booted it and XP. When Vista Final came out, I still had the dual boot but spent less and less time in XP as the Vista drivers were updated, and eventually removed XP altogether (Can't STAND having to use it on one of the computers at work. I hate XP at this point

)
When Win7 Beta 1 came out, I dual-booted with Vista for about three months. Then I pretty much totally stopped using Vista, and just used the beta (I'd occasionally load up Vista for one or two apps). When Win7 RC1 came out, I had completely stopped using Vista. I haven't run anything except for Win7 on this machine since RC1. Obviously, I'm running the retail version of 7 now

(And yes, Iris runs fine on 7 as well as Vista)
The main benifit, of course, to 7 over Vista (or XP) is that the memory handlers are more efficient (The % may look higher, but Windows actually HANDLES swapping things in memory better), the mutli-core support is MUCH better, and Win7 supports SSD and memory cards as performance enhancements *MUCH MUCH* better. Also, all of the general hardware improvements in the last three years are handled by 7 better.