Just simply which would perform better in games.
the 660 sli will be better in games that are good with sli but in some games the 280x will win just look into the games you play and if they support sli well. Look at this review and just compare the 660 sli to the gtx 680 or 7970 to get an idea of the performance. Also if you have the 660 already and are looking to upgrade a second one could be a good option but if you are building a new computer go for the best single card you can get.
http://www.ocaholic.ch/modules/smartsection/item.php?page=0&itemid=844
Well, the answer isn't that simple - but to put is simply: 660 SLI.
This depends on scaling however, and whether or not your game not only supports SLI, but is also optimized for it. You'll also have to be sure you have proper cooling and power. So, for questions like these, it depends on the games you play, and at what resolution. I'd wager that a single 280x will give a better experience at 1080p since it has the horsepower to max out every game at 1080p without having to worry about SLI/CFX support/optimization.