I think Artix-7 would be the best one/most cost efficient to mine BTC on, Virtex is a bit of overkill, and a lot more expensive.
Zynq would probably be amazing as well though (I think)
It seems to be a lot harder to get bitcoin miners to place and route successfully on Artix-7 than on Kintex-7.
I don't think it's going to be that much of a problem, If it is I can/will use the Kintex, but trying to keep it as low cost as possible.