Sigh. This shouldn't even be a question. You take KD all day. Top 5 player in the league or top 5 coach? Are you serious? Yes, the spurs might not have won without Pop, but every championship team would not have won without their star player. Period.
The question is, would the Thunder be a better team if they lost Durant in free agency, and got Pop as a coach?
My argument here is that there are a lot of star players in the NBA, but there are actually very few truly elite coaches.
My other argument is that coaches are much more dependable than players. You don't every (at least very rarely) need to worry they will miss games due to injury, or walk in free agency. If a coach is happy and his team is competitive, he's probably going to stick around. Players on the other hand walk all the time.
Coaches also tend to outlast players, because you can be 70 and still coach - but players rarely remain effective past the age of 35.
How many star players make other players on the team better? The great coaches makes EVERY player on the team better.