It's simple.
Windows 7 is no more than Vista, with a little different design, some removed programs, and which basically went back to Xp's speed, and improved it a bit. So it's just a little faster than Xp, (And I don't care about what tests say, I'm talking about my experience).
So, Snow Leopard will make Leopard even faster than it is. Which means, it'll certainly be faster than Windows 7.
That's all about speed.
And about all the other things, well, it's just like the previous versions:
Speed/Performance: Snow Leopard.
Security: Snow Leopard.
Stability: Snow Leopard.
Design: ?, we still don't know how the Snow Leopard design will be. Windows 7's design is basically identical to Vista, so I'd give this to Snow Leopard.
Best 64-bit system: Snow Leopard.
So for what is Windows better? For...em...nothing, except if you're looking for the cheapest computer you'll use only and only for games, nothing too serious.
Oh, and you can customize Windows, there are thousands of horrible, disgusting themes you can find online!
Have fun.
The only problem with Vista is it's resource intensive.
And because it's unstable. And slow. And easily infectable.
It's slow because you don't have enough RAM.
BS, it naturally gets slower the more you use it, the more you install program. Just like any other Windows version.
And it's crashing because you don't have the right drivers.
BS again. It's because of the instability. The registry. Partial uninstallations. Malware. It's not just about drivers.