I don't see how they're mutually exclusive. I use union types prior to typeguards to end up with ultimately checked, cast values. Say I have a boolean fetched from JSON as "1" or "0". By the time I expose it to the rest of the code as part of a Record, I want to change it to an actual boolean. At first I'm going to treat the inbound value as a union type, e.g. (String | Number | null | undefined | Error). After I deal with the error cases, I'm going to cast it (or in TS, reinitialize it as a single type, boolean) so that any code looking at the imported value sees it as definitely only a boolean without needing to have lots of different pieces of code run their own checks on its type.
No, that's what sum types are for.