I'd personally say start with either--if you're a really big fan of Hoenn and all the features that RSE brought to the table, you'll definitely enjoy OR/AS! On the other hand, X and Y introduce a lot of new things that no other Pokemon games have done before, like trainer customization and outfits, the ability to make a promotional video, etc. So it's all a matter of which you feel you'd like best c:
And of course, you'll want to get both if you're planning to catch them all~
I agree with the others, get X/Y first so you can get used to the mechanics and new Pokemon and while the postgame isn't too great it does at least have trainer customization and a huge regional Dex which leaves tons of different Pokemon to catch. You can still consider getting OR/AS and other awesome games afterwards because as mentioned by PlatinumDude there are tons of great games you can play with but that's another question.
Just play whichever one you like. Both X/Y and OR/AS are great games so it doesn't really matter.
You should at least get one of XY. Those will give you a good foundation for Gen 6. If you can get ORAS too, that's even better since they go hand-in-hand.
i would get both x/y and oras ! if not, definitely get x/y so you get all the new pokemon.
I'd recommend getting one of X/Y and OR/AS. Besides those two, there's the upcoming Pokemon Super Mystery Dungeon next month. There are also some popular non-Pokemon 3DS games to go for, like Fire Emblem: Awakening, but that's another story; up to you, really.