Ahh, I voted for Merengue! Haha, no special reason really because I like both of them - just think that she fits in better in your current lineup of villagers.
I use to really not like Coco, but honestly I'm warming up to her now. Her house is pretty cool too, both interior and exterior, and she is pretty unique looking..
I voted to keep Coco simply because she's a straight up weirdo, and she's a coconut. You'd think those would be reasons for getting rid of her, but not in this game! Lol
I would think that Coco would go better with the Island theme. Merengue is cute but Coco looks a little bit more tropical or beach-y in my opinion. She would blend well if you're trying to go with a pink/cute theme for your island, whereas Merengue only really fits with this sort of aesthetic and wouldn't translate to the tropical one if you were going with that one or any other sort of aesthetic. That's just my opinion though.