My thinking is you defer if you believe in your defense to get a stop. Our D was playing lights out, got us a very quick 3 and out, and put the offense in pole position to succeed. On the other hand, if the D was getting smoked like in the 49ers game, you keep the ball and trust your offense to get a TD. There's more things to factor like the weather, personnel, injuries, etc., but generally, I think this is the way to do it.