Helper, recommending a card based on it's overclocking ability without knowing your client's willingness to overclock is irresponsible. Unless othewise stated, it's best to assume there will be no overclocking. It's ok to mention the good overclockers as a footnote, however. There's a reason this is the general format of nearly every video card review done. At stock clock speeds, the 660 is clearly the higher-performing card.
That having been said, I am currently running a pair of Asus DC2 7850's each at 1200 Mhz core (1.22v and 1.24v respectively) from the default 860mhz, which is a fairly-ridiculous 40% overclock in and of itself. From various forum posts, it appears the safe voltage ceiling is 1.3v, and some folks have achieved a stable 1300mhz.