If you're the type of person that carries extra mags which ones do your carry with a sub compact?
The XD9SC has a compact 10 round mag and a 16 round extended mag. Obviously, one would conceal the weapon with a 10 round mag. With the exception of extra concealability and compliance with state law, why would I ever want to carry extra 10 round mags as opposed to 16 round mags?
It seems if the 1st 11 rounds weren't enough then you'd probably want/need the higher capacity mags for the rest of the confrontation.