This may help in your choice of 17 or 18's.
http://www.mini2.com/forum/wheels-ty...ml#post2840726
I personally wanted 17's but could only get the Rota's I wanted in 18". Whatever you do keep the rims light, makes a huge difference. Ditching my S Spokes was the best thing i ever did, but wish I'd specced the traction option now.
