I personally would go for the 1080. For the same hash you need 1 GPU less.
So its more 11 x 1080 or 12 x 1070ti.
With 1080 you can add 1 more GPU later without buying Mainboard, PSU and so on.
With 1070ti you can mine some other coins too.
But you asked for ZEC so my advise is the 1080.