Autocar compared the two cars:

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In the end, you can make a strong case for owning either one of these cars. If you’re an enthusiast and treasure driving, choose the 911. The Porsche is clearly more entertaining on a back road and the better sports car.
But if you’re keen on the futuristic appearance and are more obsessed with technology than dynamic appeal, go with the i8. The BMW’s blend of coolness and complexity makes it a compelling statement. Think of it as the world’s most accomplished urban sports car.
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