12 oz is good provided that your gyms bags aren't very hard. I go with the school of thought that 16 oz gloves under most circumstances lure you into feeling a false sense of security, especially when sparring. If your hands don't need them, then don't get them.

Mine are 14 oz, it's good for sparring and bag-work. Others may prefer to keep a separate pair of gloves for the bag and fpr sparring.