Playing on different teams, with different teammates, and with different intangibles.
If you think it's that simple a comparison to make, feel free to keep making it. I disagree. Westbrook isn't a leader. Maybe he'll grow into one. Make he'll always be KD's sidekick, and it won't matter. I'll say it again though, if Westbrook is the best player on a team, that team isn't going to be a championship contender.
I don't know about best player but Westbrook would be the best athlete on about any team he played on, including an NBA All-Star team. I don't get why people don't like him. It is the combination of the players that makes OKC a good team. Westbrook is a key part of that combination.