Dont worry cry babies Midnighter gets the Door.
You've really got a mancrush on Midnighter don't you? They say that when someone acts like they don't like a person, it's because they really do.
Anyway, I think (surprise) Midnighter wins. If Darwin adapted offensively I'd say him, but at the moment he's strictly defensive. Midnighter may not be able to straight up kill him, but I don't think Darwin will be able to kill him either if all he's doing is adapting to defend himself. Maybe if the fight really dragged on one of Darwin's adaptations would give him something he could beat Midnighter with, but that's just a guess. Midnighter could use a Door long before that and send Darwin to another dimension. It may not kill him, but nothing says it has to be to the death so I think it counts for a win. Darwin may adapt to defend himself against whatever he finds wherever he ends up, but just being in another place isn't an attack that hurts him so he's not going to adapt his way back to Midnighter.