Palladium typically has wider price ranges and therefore greater price volatility, which presents both greater risks and opportunities. Per cubic inch, platinum is significantly (75%) more dense than palladium. As such, platinum would be more efficient to use in the chief fabrication of autocatalysts, if it were not for platinum's higher scarcity and cost.
Overall the choice is yours, so be sure to contact a Monex account representative to learn more about investing in platinum, palladium, and which one could be right for you.