I don't like flip flopping the starter, so I'd probably stick with Luke. I do think there's room to play both though - you've got to be smart about it and not kill any momentum one of them is building, but if it truly is that close both should play.
Whoever is playing, I expect it to be our best offensive game this year just by nature of not playing a good defense. So it's actually pretty big for perception of who should be playing - if one guy gets significantly more playing time, the media narrative is going to be "the offense is finally turning the corner with so and so at the helm!" even though the other guy would likely have played at a similar level.