Cone Burr for a Preciso and Virtuoso. Replacing the ring burr at the same time is recommended.
Slight rust on new Baratza burrs is fairly common with the high-carbon steel used. This steel is prone to developing a small amount of rust even through exposure to humidity in the air. The trace amounts of rust can be removed with a metal bristle brush, or will disappear after a few pounds of coffee passes through the grinder and is not harmful.