I think the leather looks great, but I think the Alcantara looks the most racy, and as Stuntman pointed out, the race cars themselves have an Alcantara dash. I've liked the look in previous cars I've had from Lamborghini and McLaren.
And honestly? The fact that the carbon sill caps are that...caps. If the circumstances were such that the actual carbon sills were the structural tub of the car, I would have been more moved to order them, but that's just not how the car is constructed. I think the carbon looks nice, and I would certainly have gotten it if I could retain the Alcantara seats and dash, but that was not possible. If you want the extra carbon, it's coming with a leather seat and dash.