Originally Posted by footie
18" alloys are the best choice for not only comfort but offer a smoother transition as grip is released from either the front or rear, ie. they are more predictable. Steering feel is also better with the stock rims+tyres, the car will feel less darty which works better on slippery conditions.
Difference will not be that great, BMW would never allow optional 19" as they made driving conditions worse. If there is any difference, it will be hardly noticeble.
There are also M3 driving around with 20" (M-Bitious). He clearly stated it drives impecable..............