It looks as you have done some research allready.
I think you'll need to list the requirements off your design a check it off with the specifications of both zynq and Kintex. Have you checked availability of both devices for production quantities.
Having IP available for the Kintex seems important for you. Try to qualify this importance: suppose you want to use the Zynq, how will you implement your design, how long will it take, how many engineering resources will you need to assign, ...