Who Makes More Money? Technical Writers with Language or IT Skills?

Kai raised an interesting point about which skill (writing/technical) takes longer to master. Knowing how to structure and present information to users? Or knowing how to use a product or application? That got me thinking. If you want to make money as a technical writer, which area should you focus on? Sharpen your writing skills or deepen your technical knowledge, for example, learning how to document an API?