I've haven't done any autocross events, but in general as far as tire size, the 18's will be a little less expensive and offer more sizes. As I mentioned in another post, I think 18" is the sweet spot for performance, and 19's for looks. Not a huge difference but still a difference. If you plan to upgrade to bigger brakes in the future as you mentioned, I would watch out for clearance issues with the 18's. It would all depend on your setup. Wider tires is gonna effect the handling more than anything. The Enkei stock 18's are fine for autocross by the way.
I would buy these for street use and slap racing slicks on the stock rims.
