Left or right?
Right, by far. Has a better body, personality, and is the only character to not go batshit insane.
Rena is cute though
Rena will do everything in her power to make sure she satisfies you. She's completely nihilistic, cares nothing about her life and would do any fetish you desire so long as you're happy.
Mion will slowly develop her inner femininity and clumsily try and satisfy you until you're the only thing she thinks about.