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Macbook Airs have chipset-integrated Intel video, which have been experiencing issues on recent SketchUp versions, to the point that on PC, the 32-bit edition must be used instead of the 64-bit.
General consensus is, if the platform supports Nvidia video, either built-in or with expansion, get it. Note that AMD/ATI video has also been problematic for many endusers.
IMHO, generally speaking, hardware-wise, Macs cost twice as much as comparable PCs.