I want to buy a Mac, since I am kind of sick with my Windows laptops. I will need to use Windows though occasionally due to some apps, but I hear Windows 10 works better on Mac than Win 7 used to. My choice is ONLY between the standard 15 inch models - Iris Pro/256GB or Ati/512GB. I cannot configure it due to where I need to get it from. So my main question is does the Mac under Windows 10 work only with the external GPU, meaning that I would be getting like 2-3 hours max under Windows? I do not need a dedicated GPU, my concern is that 256 GB might not be enough for the dual OS.