Thing is, I don't know how to measure which is "more" ruinous. Either one can make for a miserable play experience.
On the other hand, you can often work around a bad player. I've had a player I didn't care for in a game I was also playing in in the past - I mostly ignored her, and the game was then usually pretty decent. It is hard to work around a bad GM.
But then, I notice you list "mediocre" specifically for the GM. Mediocre, by definition, is not particularly bad. If I'm comparing a really bad player with a mediocre GM, I'm not sure that's really the same.
So, in the end, I don't think I know how to answer the question.
On the other hand, you can often work around a bad player. I've had a player I didn't care for in a game I was also playing in in the past - I mostly ignored her, and the game was then usually pretty decent. It is hard to work around a bad GM.
But then, I notice you list "mediocre" specifically for the GM. Mediocre, by definition, is not particularly bad. If I'm comparing a really bad player with a mediocre GM, I'm not sure that's really the same.
So, in the end, I don't think I know how to answer the question.