It depends on really who else is on the team. Neither one is THE guy, but an other piece.
I think Green would be closer to be THE guy then Smith. Smith is better at defending and rebounding, but Greens offensive game if get can get consistent(still thing he's still recovering mentally) is better, not stat wise, but what he can do.
He can post well, hes a better ball handler. Smith is a good passer, maybe a little better than Green. Green is a better shooter at any spot on the floor.
Plus, how many clutch shots has Green made this season in the fourth. Not just game winners, but shots that keeps them in, or gets them the lead? And some of those are 3 point shots. Thats some thing Smith can't do very well.
So if you can put some one on the floor that can defend the paint and rebound with Green, I say trading him for Smith would be a bad move because you lose your diversity on the offensive end, and the ability to spread the floor.
I think I'd need to know who would be the SF, (and can't say PP since he's close to retiring) that you would have play with Smith.