The Mac App store really has no chance against Steam when it comes to games. It is a better platform and not tied to a single operating system. Never-mind their approval process not being so finicky.
But who's got more advertising and marketing power? Apple deserves a lot of credit for its high quality products, but people seem to forget they also sell a lot because they sell hard.
I find Steam to be unstable and frustrating on my Mac. I would certainly prefer to use the Mac App store.