Not knowing how many magazines you are getting with it, I'd say more get your mags followed by more ammo
If you plan to carry the piece and you decide to carry an extra magazine on your belt, you may want to look in to a way to carry the magazine.
I use a generic nylon multi-tool nylon pouch with a velcro flap for the double stack .45 magazine when I tuck in my shirt and when my shirt is not tucked in, I use a mag holder from kytexgear.com.
Enjoy.