Joe Harris is a nice player and the Nets just gave him a nice raise to keep him. He will be perfect for the new Nets. He can spot up and take advantage of the defense adjusting to Irving and Durant. Really a perfect fit. I suspect they are happy with him and would not trade him for Smart.
For the Celtics though, I prefer Smart. Harris would be a nice SF off the bench, certainly a need, but I don't see the fit with the starters. Smart can be deployed in so many ways to help a team. I see no chance that this trade would even be considered by either team.