It's the same in that both players exercised the rights as a free agent to go to whatever team they felt fit them better professionally...but different in the context. Fairly or not, Kevin Durant is looked at as joining the team he couldn't beat when he was up 3-1 on them. LeBron didn't join the Celtics, who eliminated the Cavs the season before he joined Miami.
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This is still massively overlooked by all the people like Paul Pierce who says Durant is the best in the league. Durant wasn't even the best player on OKC last year.