The whole reason Ainge moved Bradley is because he knew he wouldn't be able to pay him when he hits free agency this summer.
With Smart and Bradley both being free agents this upcoming summer, it comes down to a.) who can help the team more this season and b.) who will be more likely to retain.
The answer is Smart to both of those questions. He is a much better all-around defender than AB, and, if recent rumor is correct, will be much more affordable this offseason.
Oh, and to hell with helping the Cavaliers.