The MBA display leaves a LOT to be desired - it's abysmal really. The lightness and battery life are great but that display just kills the whole proposition for me, it's too similar to that of my white MacBook from 2008, i.e. a huge step backwards.
If you want to game at all with decent settings / framerates, you'll be needing a MBP-15 of either regular (2012) or retina (excepting the newest base model) variety as those are the only ones with a dGPU. You'll be disappointed with any of the iGPUs, including the latest and greatest Iris Pro. People will probably dispute that, but they won't be people who play any recent games with any regularity.
Yes the old uMBP-15 is heavy, but it's still a 5.5lb notebook. In the larger scheme of things it's not terrible to carry around, unless your bag also happens to be full of textbooks, in which case every ounce of savings counts.
Good Luck
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