If I'm in your shoes, I would take the 12mm. I would take the F2.8 over the 3mm focal length difference.
Maybe you can take a step back and think it from another view.
IF... you have not buy these 2 lenses, they are both FREE (well basically same because of the promo), and they are sitting in front of you RIGHT NOW and you can choose only 1, which one would you choose? Ask yourself why you choose it instead of the other one, then you will have your answer.
OR...
You can put both to the B&S at the same time, and see which one got picked up first, then you keep the other one ;p