1. A world in which God exists is better than a world in which God does not exist. (Premise)
2. Better/worse relations are only veridical if the referents of those relations possibly exist. (Premise)
3. Hence, God possibly exists. (From 1 and 2)
4. Therefore, God exists. (From 3 and S5)
The inference from (3) to (4) entails the usual unpacking that the ontological argument provides. I'm having a hard time coming up with any good objections to either (1) or (2), but maybe I'm overlooking something.