If I wanted to role-play, I want my homegrown marines from my colony resources. (I want everything homegrown, with no dependency with the core worlds.) However, as someone with shameless munchkin tendencies, I simply buy most of mine from the zombie pirate bases (where they cost about 133 credits a pop, less than my colony's 200). I may buy a few from the bar encounters of unstable markets as well.
As for the topic, if we had such a thing, I would grab the ones with the highest attack bonus for raiding, or whoever is the best at stealing blueprints and special items.