LeBron is a better defender than Durant, but Durant is actually underrated in that regard. He has become not only a competent defender, but actually a good and reliable one. He does have solid help from teammates on defense. But I'm betting that most people don't realize that statistically Durant is putting up just as good a defensive season as LeBron, and is a tick better based on advanced defensive metrics.
LeBron
6.9 DRPG
1.63 SPG
0.85 BPG
102 DRtg
2.6 DWS
Durant
6.9 DRPG
1.55 SPG
1.22 BPG
101 DRtg
3.2 DWS
Now steals and blocks don't necessarily equate to good defense, as we've seen in the past with guys like Marcus Camby and Allen Iverson. But that's why I also included the advanced metrics (from basketball-reference). Again, Durant is surrounded by some very good defensive teammates so that skews the numbers some. But it does give some insight on the fact that Durant is actually underrated defensively. No, he's not as good as LeBron defensively. That's not what I'm arguing. But he's better than what some people claim, and at least statistically, he's doing his part defensively for his team as well.