I’m writing a novel about two central characters. How do I decide which to use as my viewpoint character?

from The Beginning Writer's Answer Book, edited by Jane Friedman

If you’re using third-person point of view, you can include the viewpoints of both. When both of your central characters become viewpoint characters, you can more easily develop subplots and suspense, as well as the personalities and histories of those characters. Care must be taken to switch viewpoints only at scene changes—never within a scene (unless you’re an absolute master).