Both are pretty mediocre. They're both usually chance based, so you can't really rely on any of them in critical situations. If you're relying on Sol to heal to avoid dying, you;re probably dead. If you're relying on Luna to defeat an enemy...well, you'll probably be able to do it, but you'll be in for a rough time if it never procs.
I went back and forth on which is better based on random procs that you aren't relying on. I guess in that case Sol might be better since it might save you from having to expend a healing staff or item later on. But then again, you can goose your odds of getting a luna proc by focusing on a static target like a boss, who is probably who you'd want a luna activation against most. It's hard to say, but I feel like Sol would be slightly more valuable.