I posted this (and more) over on my stroke recovery blog, Stroke Of Grace, today, but wanted to share it here as well.
I think the difference is, while they were both remorseful, feeling the weight of guilt and the anguish of denying Christ, only one was repentant and believed/proclaimed that Jesus is who He says He is (God in flesh, with the power to forgive/erase sin and redeem lives). Like the thief nailed to the cross on Jesus' right side, it was in this act of believing, of trusting that Jesus was more than just a "good man" (or "innocent man" as Judas confessed too late) but that He is God, that made all the difference!
But my life is worth nothing to me unless I use it for finishing the work assigned me by the Lord Jesus—the work of telling others the Good News about the wonderful grace of God. - Acts 17:24