Why are you upgrading?
Others have told me that the sl3 is stiffer than the supersix.. otherwise, i love my supersix with the campy stuff. would love to hold out until the eps became more reasonably price, but was getting a great deal being offered to me on the DI2, dura-ace. So your recommendation would be to stay with the SuperSix?
Stiffer? Phooey. Based on what? You have to consider that a) it's doubtful that "others" have seen actual testing figures for both frames indicating that one is objectively stiffer than the other and b) even if one is stiffer, by how much? It's all about orders of magnitude. My road bike is loads stiffer than my commuter, but only slightly more so than my cross bike. Just being stiffer doesn't mean much. You'd be amazed how much of it is in your head. This I've learned doing the background research for a double blind study I'm planning to do on component stiffness vs. rider perception late in the summer.
At the end of the day, the difference between the two is probably minimal at best. If you like your supersix, then stick with it. Save the money for the next bike, or some super nice wheels, or a vacation or something.
There's this thing in engineering called "six sigma." The idea is that you try to design a product that meets the spec 99.99966% of the time. For companies outside of the medical industry, a lot of products are designed to meet "four sigma," which means it meets the spec 99.379% of the time. As I understand it the cycling industry works this way (for big companies at least) So the supersix and SL3 are both designed to meet the needs of 99.379% of all riders in terms of stiffness. Statistically speaking, unless you're a burly track sprinter putting out 2000 watts, you'll fit inside the four sigma. Which is a long way to say that in bikes of this level any difference in stiffness will probably be all in your head.