The text of Elder says: "+2. You may play an Action card from your hand. When it gives you a choice of abilities (e.g. “choose one”) this turn, you may choose an extra (different) option." The "e.g." in the parenthetical (which is short for "for example") says to me that "choose one" is not the only wording by which a card may give you a choice. If Elder worked ONLY on cards that said "choose one," it would be worded like Patron, which says that "using the word 'reveal'" is the only way a card causes you to reveal it.
Thus, I think Elder should apply to Barge, which offers you a choice with the word "either": "Either now or at the start of your next turn, +3 Cards and +1 Buy." This is plainly a choice between "now" or "at the start of your next turn." Can anyone rationalize why Elder does not work with Barge? If the point is that Elder ONLY works with "choose one" phrasing, then I would propose "e.g." be changed to "i.e." (meaning "that is") or to the Patron phrasing of "using the phrase 'choose one'."