I’ve been taught to use the em dash to set off a point of emphasis, disagreement, or a caveat. So I parse your first example as: My wife would like tea; I would prefer coffee.
In modern usage, you could probably substitute an em dash. But, if a writer did that and I were editing, I'd probably go with the semicolon. A period would probably be fine too.
I usually think of em dashes as a substitute for parentheses.
