I'd wait and check out our flex rankings when they come out, if they aren't already. But I think Cooks is a little safer at least at the moment. Sankey could get enough touches to play as a flex but I think we need some more info first.
Agree with dystone
I am no expert, but as of 8/28/14, you have to go Cooks because Sankey isn't even playing with the starters yet.