Caliber choice for first AR / SHTF

First of all, I apologize if this topic is too repetitive on this sub. Mods delete if necessary.

I am planning to purchase my first AR. I am thinking an AR 15 chambered in 5.56 NATO as it will be more cost effective and I think easier to learn with than an AR 10. My primary use will be training and recreation. I live in an urban area.

With that being said, in a SHTF situation, would you pick up an AR 15 or AR 10?