gtx 780 or r9 280x cf msi

Given the choice I would always go for a single powerful card over a pair of less powerful cards. It consumes less power, produces less heat, installs into smaller systems/cases and plays better with games.

Instead of a pair of R9-280s in cf I would be contemplating a R9-290. I have got to say, the R9-290 would probably win over the GTX 780.