I don't have the Bard Class Deck, but based on your description I would say that the Retainer would work on checks to defeat Barriers which are typically non-combat. The Dilettante would not help you with Barriers which is why it has the basic trait, and the Retainer doesn't.
edit (thanks Sandslice): mixed up the two cards when replying. Dilettante works for barriers, Retainer doesn't. I'd rather have Dilettante.