Which is worse?

This question suddenly popped out in my mind as I listened to Pastor Mark preached on Matthew 6 last Sunday. I'd really like to know what are the different opinions on this. But of course, in the end, it is the truth that I am interested in :) What do you think?

Q: Which is worse?

A. Not believing in God.

OR

B. Believing in God and going against Him (just like the devil himself).