The SoE generates more XP per hit, but has a much lower rate of fire. The staff of frost can rapidly generate multiple hits with a single activation. So it is not true that the SoF is worse for skilling-up than that the SoE. They are about the same.

Each of them also need some player skill. With the staff of frost, you want to learn how to dish out just enough hits to do the job and conserve eitr. Another player skill is dealing with multiple foes by freezing each of them with just a touch of frost. "I'll get back to you later." With the staff of embers, you have to both lead and loft your shots to score hits.