Which version are you getting?
 

So, there is confirmed to be a huge difference between the two games, designed for people with varying schedules to select whether they'd mostly like to play during the day or night. Sun runs on the normal clock, while Moon runs twelve hours displaced.

Minor differences include the cover legend, and the totem Pokémon are different as well.

So until now I was leaning Sun because the legendary looks cooler, but this made me reconsider for a minute. Interesting use of the opposite version mechanic, though.

But if, for example, you wanted to play Moon like a normal person, couldn't you just set your 3DS clock back twelve hours?

I assume so, but given how many 3DS games chastise you for messing with the clock for at least a day, you'd also have to deal with all your other games being 12 hours off, except perhaps Animal Crossing: New Leaf, which has its own clock offset you can set at the opening menu.

I'm getting both games. :D