I know this isn't the answer you're looking for, my back-up would be either of my 1911's or a full size M&P. My primary would be an AR. Hope it never comes to that, because the S would've had to really HTF. Till then, one's enough for me.
I thank all of you for your opinions and ideas. For those of you doubting I can carry both guns- people said the same thing when I said that I was planning to carry a Full-sized 1911 in the summer. I actually have pants with very deep pockets (one of which I can and have pocket carried the 1911 in) so pocket carry isn't truly off the table. I'm also considering using my coat pocket in the winter. Does anyone choose to carry in this manor in winter and how practical is it?