I'm too lazy to math it out but Luna will likely do more damage in the long run since it procs twice as often. While it might not let you OHKO units as much, if you were going to wipe out a unit in one attack with Astra, that unit was probably so weak that it doesn't matter if you can't OHKO it or not.
http://img.photobucket.com/albums/v36/qwertyman/caterpie.gif Hey look, it's qwertyman2002, let's read his posts!
Ignis is equivalent or better up until lategame hard stuff where Luna pulls a little ahead, but honestly luna/ignis are similar enough damagewise that it doesn't REALLY matter which you pick. They share excellent proc rates = reliability, probably the most important stat for a proc skill to have; Astra is half the rate and gets worse against high defense opponents where Luna gets better. Since high defense opponents are going to give you more trouble anyways, Luna's bonus is both more useful AND more reliable. Easy choice.
Another day, another unsuccessful genetic abomination!