I'm going with Moon for sure. I'm not really sure why, but I tend to prefer the flying legendaries to the ground ones. It's the same reason I picked Y over X.
As for starters, it's hard to say at this point. In general, I tend to lean towards the water types (Gen II, V, and VII) and fire types (Gen I, III, IV) on initial playthroughs. I don't think I've ever used a grass type during a first playthrough, but I could be misremembering. So for that reason, I'll probably pick Rowlet, even though it's my personal least favourite of the bunch design wise. That could all, of course, change once their evolutions are revealed, but I feel like I neglect grass types a lot with only like 2 exceptions, so ya.