Oz would definitely be more STABLE on Emerald, I think. Since they're a bit less emotionally dumb than Ruby is in terms of welcoming new people. But they'd also probably encourage his self sacrifice thing which would put him at odds with them at times when he realized that hey no wait I DON'T WANT TO SELF SACRIFICE. Also he would be a drunk.

Mytho would probably be lolz WAY MORE MORALLY AMBIGUOUS on Ruby. Like. Assuming they wouldn't have Ahiru, Ruby would probably set some very very questionable guidelines for Mytho initially and also would probably not discourage his raven side when it came out. So he'd just be like "Yay murder and revenge. :D" and it'd be boss.

Break on Ruby, Mukuro on Amethyst. go.