Lee is a better offensive player. Bradley is a better defensive player.
So the question becomes, is Lee's offense that much better than Bradley's, and is Bradley's defense that much better than Lee's.
Both are great defenders. While I think Bradley is better, I'd have no problem if Doc decided to have Lee guard a Kobe or Wade with the game on the line.
On the other hand, Lee's offense is superior to Bradley's. Lee is a legit three point threat that teams will not want to leave open (despite his poor shooting this year). Bradley is great at cutting to the basket, and his jumper looks better, but I don't think teams are as worried about him on offense as they would with Lee.
If we judged the two on these comparisons, I'd say Lee's better AS A PLAYER. In terms of value to the team, though, I think Bradley is more important.