I'll be picking up both versions, so I'll probably get to use a couple of starters, but my main one (probably on Moon) will be Popplio. I've used the water starter for my main playthrough of every generation, and I like tradition, so I assume I'll be sticking with that. I could analyze the starters to death and worry about which one to get, but ultimately, I know I'll settle on the water type anyway, so there isn't much of a point. And I can get the other starters later from trading regardless, so it's not a huge deal to me.
It's interesting that Popplio is so unpopular right now, though. I don't know if it's my favorite appearance-wise or not (it's definitely not my favorite water starter, and I'm a fan of Rowlet, too, but I like it reasonably well so far). I wonder how things will change when the evos come out (like how a lot of people liked Fennekin, but ditched it when Delphox came out). A couple of people commented that finding out about evos ruins the surprise, and that may be true for some people, but I've found that I don't like to be surprised by an evolution I don't like the look of (case-in-point: using Totodile for all of Gold because I decided that Croconaw and Feraligatr looked ugly when I saw them on a friend's game). Now, I've found if I spend some time to get used to an evolution that I'm unsure about, I'm more likely to like it (I like Feraligatr better now than I did nearly 16 years ago, for example), so I find early exposure to starters helps me, personally. But it's been so long since I've played a game unspoiled (since Gold, since I bought guides to read with every game at launch since, and now I enjoy reading about Pokemon on the internet a little too much, even if I don't need to do so to get through the game; I simply like doing it) that I don't remember what it's like to do so. I don't think the excitement would be quite the same as when I was younger, because, well, I'm older, and I know more. I'm also too much of a completionist and a planner (though I try not to come up with my whole team before I start playing a new generation), and I want to reset for good IVs at key moments, so I like to know when those moments are likely to appear in advance (and I like to know what stats to optimize). Yes, I know, in game IVs don't matter (except for legendaries, which you can't breed), but I still like to try to make my team at least somewhat decent for my playthrough.
So, Popplio for my main file on Moon. Perhaps Rowlet for my Sun file. Litten doesn't do it for me so far, but I'll see what the rest of its evolution line holds.