No question about it, the gtx 460 768mb is definitely the faster card and the better buy.
Google it, check toms charts, check anandtech etc. you will see it is a good amount faster. And the 460 has a huge overclocking head room, its stock clocks are relatively low at 675, I used to have a 460 768mb and had it at 920mhz. It was nearing gtx 470/560ti levels and it was still quiet and cool. As long as you arent planning on playing 1080p with full anti aliasing it is a great price/performance card.
EDIT: to clarify you CAN use the card to play 1080p but you may run out of memory since it is only 768mb if you turn up all the anti-aliasing options. But regardless it is faster than the 550ti which has more memory but wont really use it as well since it is slower.