Which is a better motivation for being a "good" person: believing or not believing in God?

I ask this question because it seems to me that a good case could be made for either.

If you believe in God, your motivation is to try to live your life according to the wishes of the being that created you. The satisfaction comes in knowing you fulfilled your function in this life. Additionally, you will be rewarded in some way in an afterlife.

If you don’t believe in God, your motivation is your own humanity and the brevity of your life. Since you have so little time, you want to make your time as enjoyable for yourself and others while you are here. I might be mistaken, but I think Socrates taught that “doing good” was beneficial to oneself as well as others. (I’d be grateful if someone who knows about this could expand on it a bit.)

So the question is: which makes a better argument for why a person should be good?