How do we decide which one to keep?

A: The situation is just like resolving the two Goals that result from Double Agenda: whoever got rid of the Rule allowing the double Goal/Ungoal situation gets to decide which to keep, and which to toss.

UNLESS Cosmic Agenda is also in play (which forces you to play UnGoals). In that case, you should be forced to keep the UnGoal in play, and trash the Goal, because this is analagous to having to play the UnGoal.