The D will RushYa, Like Moscow
What makes this a tougher question than it appears is the fact that I have seen Durant improve every year in all phases of his game, including his defense. He is not a bad defender, his length really makes him a solid one on one defender. Where he struggles some is in regards to team defense/help defense . . . he's not bad, but it is the area he needs to improve the most.
That said, the man is only 24 years old and he is the best offensive weapon in the game and arguably the best shooter in the game. He certainly isn't a bad consolation prize for any team.
But Lebron = Magic Johnson, except stronger and faster . . . and that is ummatched in the NBA. His intensity level rivals Michael Jordan's and he only just turned 28 himself, with a body built to play into his mid 30's before we see any type of real dropoff. His improved shot and his improved post game, combined with everything else will probably make him the greatest of all time when it's all said and done.