Personally I tried AoC with both Vista Ultimate 64 bit and XP Media Center 2005 32 bit.
I think the Vista ran better for me. Not sure if it's because I have 8 G of RAM under Vista or what.
There where however some people saying that the BETA was running better in both Vista and XP for different people. It seemed Vista was better on the medium-high to high end PC's and XP on the low to Medium end PC's.
Just what I noticed in the global chat when I paid attention to it.
I'd tell you more of what people keep saying in the global chat, but I turned
it off after I kept seeing the same shit you see in every BETA.
Hope this helps in your decesion.
(And shhhhhhh don't tell anyone I told you!)