Presumably this will be a 997, not the current 991? Even so, it is much more fun to drive than a 6 Series. So depends what you want it for. If you want it for performance driving, get the Porsche, even though with the top cut off it is not as sharp as a coupe. If you want a top-down cruiser for the summer that's also relatively civilised on the motorway, get the BMW. I think the BMW will depreciate more heavily.