Which should I get and why

[Fantivity]

Kinda skipping to the box here... Now I'm not sure why you didn't list X or Omega Ruby. But as the first person said, if you want to start with the new features of Generation 6, start with Pokémon X/Y. Alpha Sapphire/Omega Ruby actually looks good but I recommend getting the demo first because you can get rewards to transfer to the full game. So I'd say get X/Y first, get Apha Sapphire/Omega Ruby later.

I didn't mention omega ruby or X because they have about the same pokemon and if I wanted I could get the pokemon that aren't in Y and Alpha sapphire through trade as I've said before I know how to use most of the features already through the demo and videos .