Hmm, I gotta say females. My first fish (well, my first serious fish-keeping fish, that is) were a male and female betta. The male died of fin rot and fungus. The female is still here - 11 months later! I got a replacement male and came home one day to find him dead, his nose wedged in the gravel. (how'd THAT happen??) Finally, when I got my 55 gallon, a male betta came with it . . . and when I returned from my 6 week vacation, he was MIA. No idea what happened to him. And the female had actually been chasing him around instead of vice versa . . . ! But the female's been here from the start, and several people have told me, "no way, he can't be a girl, he's too pretty" about her. Striped and purpley and violety. Very jeweled and with a definte personality. So I have to say girl power on this one . . . at least they survive!