You would have to prove that God's existence was a logical necessity. It would have to be almost as solid as an axiom. Can you do that? Prove God's existence using logic, please. I'd love to see it.
Evidence is better. It's difficult to formulate premises that everyone can agree on. In order for your "ontological" proof of God to have any ability to convince others, you have to get them to agree to your premises. With empirical evidence, there's not much room for debate.