Here's a decent article showing some aspects of the differences:
http://www.caranddriver.com/features...d-tires-tested
The main reason to go with 19's or up is for looks (unless you need them to fit unnecessarily massive brake calipers).
18's will be able to provide better ride quality, similar grip and handling, have the potential to be lighter, access to much cheaper tires and IMO look just as good if not better (going with too big of a wheel without the huge brakes to fill in the space doesn't look right to me).
I think 18's with 245/40 tires are the sweet spot for the F30 chassis.
For more info about how increased wheel and tire weight effect the car:
http://hpwizard.com/rotational-inertia.html