Was it not for my SlipStream 800 rpm's which were on a discount (bought for approx. 7 USD each) I would probably have went for the AC fans.
If I had the chance to buy 800rpm Slipstreams for that amount I would have bough them instead of the F12's as case fans because I wouldn't need to change their voltage. For the CPU fan I would have always gone with a faster fan to keep the temps in check and to be able to overclock the CPU. As far as I know there is no 800rpm fan (or similar noise fan) on the market that would keep my CPU cool with that heatsink that is highly variable and dirt cheap (dirt cheap is a bonus).