What are you trying to accomplish with these laptops? I wouldn't guess you're looking to do gaming, because that is still done mostly on Windows. And if you're not gaming, that defeats most of the purpose of getting a discrete video card.
Also, in addition to the video card, there's more differences between the systems. The Mac uses a faster SSD while the ASUS uses a slower hybrid drive. The Mac uses a lower i5 while the ASUS uses a i7.
But stricktly speaking, the NVIDIA is a slightly better graphics card. In Skyrim on low settings, the Intel 4000 in the MacBook Air gets 30 FPS. The NVIDIA in the ASUS gets 36 FPS. In Diablo 3 on low settings, the ASUS gets 89 FPS while the MacBook Air gets around 50 FPS.
MacBook Air 13 inch
1.8GHz dual-core Intel Core i5 processor (Turbo Boost up to 2.8GHz)
128GB flash storage
Intel HD Graphics 4000
ASUS Zenbook UX32VD-DB71 13.3-Inch
Intel Core i7-3517U 1.9 GHz, dual core (Turbo up to 3GHz)
4 GB RAM
500 GB 5400 rpm Hybrid Hard Drive
Nvidia GT 620M
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