Hayward is probably slightly better overall. He might be a better fit for this particular Boston team - since they were offensively challenged last year - than Bradley, the major distinction being that Hayward can get his own shot and Bradley can't.
But if I'm trying to build a title team from scratch, I think I would take Bradley. If he's the third best player on your team, I would take Bradley because he needs the ball less than Hayward (not saying Hayward can't be effective off the ball, but if he's not going to be a high usage player, his impact is marginalized a bit).
And Boston is one of the few teams in the league that kind of has the luxury of building from scratch, no? Maybe Anthony Davis isn't available today or tomorrow, but if I'm them I'd rather pay Bradley, maintain their other cheap contracts, and let the young guys develop on rosters that aren't going to be as good right away but have less of a dead end vibe to them. Building around Isaiah, Hayward, and Horford seems like a dead end.