im not 100% this is just my theory:
MASS... would be exactly that, by definition, the weight of the ship
Rotational Inertia... i THINK involves the amount of force that the ship exerts to turn and corner... or something like that... wich can also create a realistic "drifting" effect where the ship dosnt immediately stop after a rotation on a given axis
(someone correct me if im wrong on the RI)
i know that dropping the mass and upping the RI makes a ship turn, rotate, and just faster in general, and vice versa.