in most games yes it would wipe the floor with a single 290x, but cost and power consumption really do make the 290/290x the better buy.
personally if i can afford two 280's i would jump ship to nvidia and get a 780 and save about 150quid.
or if i was gonna be all fanboi and must have AMD then i would get a 290(not x) and hope mantle brings some serious performance gains on the few games that will support it this year.