Oh, the 760 is a superclocked version?
Anyway, AFAIK, it can still works fine with 2x6pin power supply, and should not draw more than 225W in total. Of course, you still better check it under your hack now if it will draw more than 75W from the 8pin cable under full load. If not, than you are 99.9% safe to put it inside the 4,1.
The advantage of the 280 is very easy to flash for the boot screen. Fail safe for flashing. And use the native driver from current OSX. However, some port may be inoperative after flash since the double D version's non standard port layout. If you need all the ports, then you basically can't flash it. That means it won't has much advantage over the 760.
And since you may not want to flash it, that means you can't lower the clock or voltage under OSX. Which also means the 280 may draw more than 100W over the 8pin supply (50% more than the 75W spec limit). This will be my primary concern. I own a 7950 and a 280, both of them do pretty bad on balancing the power draw. So, even though the 280 should draw less then 225W overall, but may be still heavily rely on the 8 pin supply.
I am not a nVidia guy, so can't give any comment about how the 760 works under OSX. But so far, it seems that as long as you can install the web driver, then everything will be fine.
In terms of performance, I am quite sure that the 280 is a better choice, but it really depends on your workflow.
In either case, if you have extra PSU, or you are happy with the PSU Mod, then both card should work well. And I will definitely keep the GT120 as my backup card.
