New Chihuahua: Male or Female?

Question simple: Male or Female for our new Chihuahua. We have two already. Duke is a 3 year old Male Deer head and Diamond is a 1 1/2 year old Female. We don't want it to be battle of the sexes! The male is a "humper" but not aggressive, the female is a complete sap but also isn't aggressive. They get along fine, but i'm a little skeptical of allowing my family bring another animal into the family and disrupt "the balance". I've heard several different stories...."Two females with get territoral....two males with try to dominate the female" etc......there is my delemia