I think what the OP is getting at is that as great as Marcus Smart is, he doesn't have the kind of quickness that Bradley has when defending guys like Curry, Irving, Lillard, et al. I love Smart but I would worry about who is going to be guarding big scoring PG's on this roster if Bradley is sent out.
But Butler is still an upgrade over Bradley, so maybe it's more important to get the better player when you can (assuming a fair deal).