Otis put his mojo right out in front. He wasn't trained how to hold the mike or how to stand on stage. He was raw, church soul. Marvin's father was a sanctified preacher so he came from the same root as Otis. But, his desire was to present himself in a less threatening less masculine manner. This was fostered by Mowtown and it was successful at getting crossover success.
Remember that Otis got crossover success with the hippie-love crowd who rejected the poofy poodle skirt and big hair style.