Depends on how and where you use the car.
Personally I prefer 17"s, which have at least a 45 ratio sidewall, because of the poor roads where I live.
Actually changed from 18s to 17s on the Audi after damaging some tyres/wheels on potholes.
If you are mainly on motorways, and other well surfaced roads, than a 40 aspect ratio sidewall commonly used in 18" fitment should be fine.
Looks-wise the it is totally subjective... horses for courses.... but... keep in mind that a 225/45/17 and an 225/40/18 have almost the same rolling circumference (less than 0.5% bigger = approx 1.5mm larger), so strictly speaking both "fill out the same". The 18" would be a little wider (about 12mm wider, so 12/2=6mm closer to the wing if using the correct off set) as the 225 is a percentage aspect ratio on the 18" (as is the sidewall measurement).