I'd recommend Flash. I think it's the more entertaining book at the moment; it's also not as encumbered by back story right now.
"You see, but you do not observe."
Based on the issues you mentioned, I would definately pick Flash, if you are absolutely set on one of those two titles. GL will definately cross over with the other GL titles at some point. While the GL history isn't too hard to figure out (and Wikipedia is your friend) there are more pre-DCnU elements in play with GL (including the ver fact that Sinestro is GL in the title and Hal isn't even a Lantern right now) than there is with Flash. Even prior to the relaunch there were'n too many stories with Barry since they had just brought him back after 20+ years, and since I don't think DC is in a hurry to mention any of his possibly COIE history, I think you'll be pretty set with the new 52 series all by itself.
The Flash, one of DC's best titles now.
I dropped GL simply because of the continuity as there is plenty of dialogue and it would probably be much more significant to me had I been a longtime GL reader. The two main Lanterns featured in the current story arc have a long and complicated history especially Sinestro in Johns previous run.
Action Comics, Animal Man, Aquaman, Batman, Batman And Robin, Batwing, The Flash, Superman, Swamp Thing