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I don't think there is a difference. I don't see a difference, at least not to the naked eye. Dude, go with the one that fits your version of the GS. it's not that hard a decision. I don't have the F-Sport. I hated the look of the front bumper without the fog lights, and couldn't see paying more money for a less attractive F-Sport when it isn't any roomier or quicker than the non-F-Sport. It didn't add up. But I do have a rear spoiler. It looks great. Get one.
F-sport one just sits atop the trunk with minimal extension on the rear face of the car. Non-f-sport extends more downwards towards the rear and has a notch to prevent covering the Lexus emblem. F sport spoiler looks cleaner IMHO, but probably isn't worth/doesn't look 500$ more (looks the same unless you look closely).