Neither of them have anything to do after you finish the main story too

I suggest wait until S&M, but if you want, go with ORAS if never played Ruby/Sapphire/Emerald or X&Y
&Z if you had played them. You can get all the pokes with one game thanks to the GTS, but it will take a bit more with ORAS since is missing 300+ pokes but it has every legendary except for the ones in X&Y.