Offhand, I would say that the meanings are indeed interchangeable, but they are often placed into different sentence structures: I like sushi rather than sashimi. I don't like sashimi; I like sushi instead. In fact, I don't think I could simply substitute one for the other in an identical sentence; can you?
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.