First up, though I'm no longer a Windows user (lies, I help support 3500 XP computers at work and approximately two Macs) I do like the look of Vista's Aero. It really is very pretty. I like to see websites and forum layouts based on the graphics particularly.
However, I'm a Mac person and I adore the Aqua interface just because it looks so clean and because I find it so easy to use.
Of course there's also the RAM issue. I think I'd see some differences if I were to put Vista on my two different Macs, but I don't see Mac OS X run any differently between similar usage on my Core 2 Duo/2GB RAM MacBook and my 2Ghz PowerPC/512MB RAM iMac G5.
I'd probably be switching Aero off if I had Vista just because I am a speed freak xD