I know the r9 280 is better... But Sli has less problems than xfire if we put 2 on the same board, wich one will win?
My pc http://pcpartpicker.com/p/3wcpc
I'd always say take sli over crossfire. If it was a 290 or higher that supported crossfire on the pcie bus instead of the bridge then I'd recommend crossfire. Take the 760's in this situation. Have you considered one good card over a multi GPU setup?
A single powerful GPU is always better in the end especially in frame time variance and compatibility. Lots of gamed don't support multiple GPUs and if they do its down the road when drivers are released for it. Take a 780 or 780 ti