The soarer, I'd go for 18%, no hesitation. It's not that dark that you can't see out of it, but I guess I got used to it (I had 18/20% on the silver GTS-t, white GTS-t and the GTR)...
But from the outside it looks REALLY dark, and I'm not sure that would suit the look of the BMW. Ideally, I'd want 25% 3M Security Film But at this stage, I'll be going for 18% again for the bimmer.