Thanks, it seems like we agree that the XPS15 with nVidia was more a workstation and better compared against a MBP and not MBA, as I've already explained a number of times.If you are looking for a laptop with an Nvidia graphics, then:
1) Any Nvidia smokes any apple graphics offerings.
2) You are talking about discrete vs integrated, apple doesn't offers add on video cards for their laptops anymore, when they did, they were more expensive than PC laptops with equivalent video card.
So the only reason to go apple laptop (assuming you don't care about the operating system/software) is the amount of hours the battery will last.
As already mentioned, as it seems you agree also, that the MBA15 is aimed at a different market where the better screen, SC performance, weight, portability, noise, lack of heat, performance on battery and battery life itself, not to mention the much better OS is a number of reasons why somebody would choose the $250 cheaper MBA15".