Thaddeus Young is a much better defender as an undersized PF than Jeff Green is. He, Young, is actually good enough defensively to be a starter there in the long term if he wishes.
Thaddeus it the more physical player, tougher, more comfortable banging against bigger guys in the post. Doesn't quit like Green frequently does when matched against bigger guys.
Jeff Green, on the other hand, is a much better offensive player. A better shot-creator.
As small forwards, I think the same is true again but the differences are smaller. Green is still the better offensive player but by less of a margin and Young is still the better defensive player but by less of a margin.