In general, I also tend to prefer regular Pokémon over Legendary/Mythical Pokémon, because the former are more fun for me to use. That being said, I like Arceus's design more than Silvally's. I think Arceus's design certainly does manage to capture a certain striking, ineffable feeling, and without being unsubtle about it. Whereas Silvally is cool, but I actually think Type: Null looks cooler with the helmet.