Spurs because the spurs would win more games without Kawhi than Cavaliers without Lebron. The Cavaliers lose more +/- overall contribution when Lebron is out than when Kawhi is out.
This is Kawhi's +/- contribution to the Spurs when he is on the floor generally. The BPM is the Box Score Plus/Minus:
So when Kawhi is on the floor for Spurs he gives them +7.9 Box Score probability. As such, without Kawhi the Spurs lose -7.9 Box Score. Fair enough right? Wait a minute. Lebron just edges out Kawhi in terms of what he adds to the Cav's Box Score when he plays (or doesn't):
Relative to Kawhi, who is a beast in his own statistical right, Lebron nearly adds a full point to the Box Score when he's off the court. He also adds more offensively and defensively than Kawhi too.