Is morality better grounded in atheism or theism?

Sean McDowell recently debated James Corbett on the question of whether morality is better grounded in atheism or theism. ( MP3 here)

Some highlights include:

• We require a transcendant source of morality in order to judge other cultures. We require a “Law above the law”
• Objective moral standards require a God
• Without objective moral standards, all we are left with are preferences.
• Any valid explination of morality needs to account for free will. Where does it come from?
• Materialism relieves us from all culpability