If you've already got those cards, I'd go with the dual 9600GT setup. If you're planning on purchasing new card(s), then you should probably reconsider your options completely.
Unless you're specifically limited to those few cards for some reason, something else, dare I say ANYTHING else, that came after those particular cards. On the higher-end scale, of course.
4850, 4870, 9800GTX(+), GTS250, GTX260... all better in not just single card performance, but dual-card as well (in most titles and moreso at high resolutions).