The 20's come with Pirelli Nero performance tires in a unique size runflat. There is actually no other tire on the market as an exact size replacement. They are useless in cold and snow. You'd best be buying an extra set of 19" rims and higher-profile tires.
One possible advantage of ordering the 20's is that you get a 155mph top speed due to the tire rating. The 19's factory-limit you to 130. If that's a concern.