Personally I would by X/Y. That said, you mentioned three games you have yet to play, so there isnt a pressing need for your purchase to happen right away. specially if you have some sort of plan to play it in order.
Also we still have no idea if "z" will be a third version, or a sequel. So waiting may just end up adding another game to your list.
.....said an almost 30 year old married guy driving a van part time for a living fc:0602-6823-9646, SV:932,FS:electric. my wifes 0146.9112.0071
It was two years between Ruby/Sapphire and Emerald, and two years between Diamond/Pearl and Platinum. Since there's no new system coming out to rush things like with Black and White I wouldn't expect Z until late 2015.