... I totally would go for the 19" wheels - I did. It is teh "standard" equipment on the M5 and BMW wouldN#t chose such equipemnt if it wouldn't do well with the vehicle.
In my opinion they also look a lot more professional than the more decked out 20"ers. The latter one to me are more of a fashion statement rather than a utilitarian detail. The tires sit nicer on the 19" rims and look a lot more like actual high-performance race tires to me.
My 2 cents, backed up by the following pictures ;-)
