I am very torn between Rowlet and Popplio. I really like Rowlet mostly because of its dual typing and its owl appearance. I keep having this thought process that Rowlet might end up having access to the perfect Grass and Flying move pool, the move pool that Jumpluff has gotten so close to perfecting despite only missing one of the moves (Fly).
However... Popplio has my favorite type of all time, Water type. That and it has that apparent growth rate of growing from something that looks a little goofy into what will most likely be a majestic and beautiful water pokemon, which was made much more apparent when its second stage evolution was revealed.
I'm still leaning more towards Rowlet, but I'm completely unsure, I might change my mind to choosing Popplio at the last possible second when starting the game like I have done a few times in the past. (I did this in OR, I kept telling myself I was going to choose Mudkip over and over again, and then right when I was confronted with the choice, I made a chaotic decision at the last second and chose Treeko instead, and I'm really glad I did.)