I don't fight because I hate what's in front of me, I fight because I love what's behind me.
I think both have things to their own. In a way, we don't know 6's complete story because of his black inked file, so we may never know what kind of crazy -blam!- he got himself into.
However, quoting Cortana "They let me pick. Did I ever tell you that? Choose which ever Spartan I wanted. You know me. I did my research, watched as you became the soldier we needed you to be. But you had something they didn't, something no one saw but me. Can you guess? Luck."
So yeah, if the best freaking AI chose him over 6 is because he has something to him, call it luck or whatever you want.
John 117 "Master Chief Petty Officer"