Botvinnik complained about his weakness in tactics while Petrosian didn't.
Petrosian performed better in Olympiads than Botvinnik. The Soviets were skeptical about even allowing Botvinnik on the team.
Petrosian was better at defending than Botvinnik.
Petrosian was better at endgames than Botvinnik.
Petrosian was better at speed chess than Botvinnik.
Petrosian defeated Spassky while Botvinnik couldn't.
Petrosian's mother liked him better than Botvinnik's.
I read that both players liked to play quietly but Botvinnik would go into complicated positions and Petrosian would keep the game quiet.