We can throw PER out since it doesn't consider defense. BPM also doesn't consider defense. I also expect their respective RPMs to be much closer by the end of the season. Kawhi actually does have a higher ORTG/DRTG spread than Curry. No doubt Curry is having a historic, historic season, but we can't just discount Draymond wholesale. They were a 51 win team before he became a starter in 2015 in which they pretty much cruised to the le. Another thing that favors Kawhi is the fact he has to carry a significant two way load every night, guarding positions 1-4 while also being the primary wing scorer. He's the only elite wing/guard in the league who has no real complementary wing/guard scorer to help relieve the scoring burden he's tasked with (Tony is that guy on some nights, but he's nowhere near the threat Klay is).
I've always agreed that the MVP should go to the best player, so I'm for Curry winning it, but if it's still going to a good team's most important player, I simply think Kawhi is more important right now, especially with Manu (who's our second best perimeter player after Kawhi) out.
Aa good as advanced metrics are, they still can't really measure defensive impact all that accurately. Defensive impact is still an "eye test" thing, and Kawhi is quietly as historically great a wing defender as Curry is historically great a shooter. I've maintained peak Kawhi is the best perimeter defender I've ever seen, just like peak Curry right now is the best shooter we've ever seen.