i guess the preformence hit depends on your computer...
AA i have 2x
AF i have 4x
i consider AF more importent. i take a biggish hit with the AA with my 6800Vanilla, 3000+ 64 AMD, 1GB mem, in games like Rome: Total War and Farcry.
I see more quality with AF then AA. i have desktop and also every game at 1024x768, the AF makes things very attractive with less a preformence hit then AA.
thats my 2 cents