Originally Posted by Transfer
Only real differences are tire cost, tire availability, and comfort.
This is correct, no real difference in terms of traction between 18 and 19. There tend to be more tire options in 18s (especially aggressive ice/snow tires). Improvement in ride quality for 18s is also negligible (even on the bad roads around here.) Coming from a guy going into my 9th winter in an M3 on 18s...