If the game mode Operations comes back in SF in a bigger way (all the maps and more events) together with Warzone then go for SF. At least if you like that kind of MP but there is no information about that part. Wait until Gamescom to decide perhaps because I think by then we'll have more info on both games.
I have the same problem but I pick KZ at launch and BF a couple of months later.
PS4 will sell 25 million units before 31st of March!
Edit: I didn't state what year! HAHA, so I'm still right in some sense