I don't see Sabonis or Poeltl as major upgrades over Kelly. He's older and more experienced so I pick KO.
I would take a bunch of guys in this draft over KO, including: the top 7 picks, Denzel Valentine, Malachi Richardson, and Dejounte Murray. Most of those teams would say no to that trade.
I'm not a complete KO hater- I just think that he's a role player, so I would most rather roll the dice on a player with a higher ceiling.