i would say X for compeditive reasons, because the legandary pokemon is better then the y one and char x is better then y and ect
but if you don't care for comp i would say get y because the pokemon look better.
but its up to you :P
oh from what i heard noivern had great special attack ._. so idk man infiltrator is a great sweeper ability and frisk is a good scout ability...it just depends on what ability you have i guess