Quoting T!M:

Quote:
... Both are labels that aren't explicitly mentioned in the crew credits table as allowed credits, but for both the case could be made that it's the same job, only with a slightly more specific title.
...

Disagree on this. Usually there is a hirarchy which is expressed by these prefixes.

If there were two equivalent art directors working on a film, there would be two entries for Art Director. If there is an art director responsible for the whole film and one only for a part of it, then there is only one person credited as art director, while the other is credited as unit art director or as in this case on-set art director.