I prefer she call him Clark. That's what he sees as his name, so it feels right
I prefer she call him Clark. That's what he sees as his name, so it feels right
I like her calling him Kal. It's an informal/friendly way for her to address him without giving away his secret identity, and I like she's the only one who really does it, denoting the closeness of their friendship.
For me it's context sensitive. How does he receive them? Does he LIKE being called Kal? Also, the situation is a big influence. Why is she talking to him?
Then there's levels of secrecy. Few people know Clark Kent is Superman. But many people know that Superman is Kal-el.