In my opinion a street only M3 can definitely wear a set of 20" wheels properly. They look right on the car. The problem is the size adds weight as already discussed.
19" looks GREAT on am M3 and you save a good amount of weight by choosing 19".
Are you the type of guy that only uses his M on the street and takes intersection turns a little faster? Than get 20's. But if you use the car on the track or are a serious canyon driver, 19 is your baby
This "everybody does it" that i see in this thread is disappointing though
. For every car purchase you should do your research, find out why "everybody does it", and make the choice that will make you happy.