I want to go into the starters' potential viability. I think all three actually have a shot of being useful in game. Obviously these are subject to change as we learn more.
I actually see Popplio as being the worst of the three. It's water typing is always useful for the required HMs, but just doesnt seem like it will have the pop of the other two. On the other hand, with the exception of extremely poor stat distributions, water typing has always been an amazing defensive typing. Unfortunately, defense is relatively more useless in game than in competition.
As is, Litten seems like a decent fire starter. It is the first fire starter i actually like the design of (yes, I'm not actually a fan of Charmander's design. Go figure.) Fire typing is a great typing, and its form makes it seem like it will have a decent stats to abuse an offensive typing. I see it struggling a little early in game, but being the most competitively viable of the three, with amazing in game stats.
Rowlet is an interesting case. I see it amazing if it gets decent offensive stats. Flying grass might be pretty poor defensively, but it is a great offensive duo, being resisted only by Steel among pure types. This means between grass and flying STAB, it only needs extremely common fighting or ground attacks to have neutral coverage. The pokedex even mentions that it attacks by kicking, which almost assures at least Low Kick.The only issue i see is that it is round, which tends to indicate a bulky Pokemon.
I'm torn between Rowlet and Litten as my starter. Litten is my favorite of the three, but Rowlet looks to be the game's easy mode. (Fly is actually a very decent attack in game, after all.)