Both are different cars, one is a NA V8 and the other is a twin turbo V8. If I were going to keep the car for a long time (6-8 years) I would go for the GTS, but if you're going to keep it for 4-6 I would do that X5M.
The current body style for the Cayanne is going to be updated, and the X5M won't be changing for a while, so for looks that all depends on you.
IMO, I would get the X5M.
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