I'd say the answer to this question depends whether you're a sensible person, or just a little bit nuts for bikes, and want to feel that state of the art drivetrain through the tactile nerve of a mechanical cable. And also, mechanical derailers look better.
I'd be torn. If we're talking disc though, the purism is broken; I'd go for the Ultegra Di2, but with the less ugly Dura-Ace rotors.