Actually I suggest you get Omega Ruby or Aqua Sapphire (Gen 3 remakes) and trade for the Pokemon you want from X and Y.
You can use the GTS to get Xernas and Yvetal.
Also ORAS has all of the non event legendaries from before XY.
Though sadly if you would have gotten ORAS and/or XY before Today, you could have gone to McDonald's and download the event legendary Hoopa.
Sadly the event ended yesterday in NA.
Just get one of the Omega Ruby (Groundon/Team Magma) or Aqua Sapphire (Kyogre/Team Aqua)
These two games have the items necessary for mega evolutions of Hoopa, Groundon (has as ORAS special move), Kyorge(has as ORAS special move), and others that aren't in XY
Both games have the all the mega evolution stones and TMs.
XY though are better for breeding shinies and for some abilities.
To repeat choose between:
Then get Z when it is announced (it's gradually leaking).
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