How do you know when to use a Thevenin equivalent instead of a Norton equivalent, or vice versa, when both a voltage source and a current source are present?

If a circuit overall behaves more like an ideal current source than like an ideal voltage source, you might use a Norton equivalent instead of a Thevenin equivalent. (An ideal current source can only have a Norton equivalent, and an ideal voltage source can only have a Thevenin equivalent. Why?) In some cases, either might be useful. Use of Thevenin equivalent tends to be more common.