"Value" is an interesting thing.
I'd say Sully has more trade value, although due to salary rules in trades, he'd have to be packaged with somebody else. I get the sense that Green probably has negative value.
In terms of importance to the team, there's an argument that Green is more important right now, just because we don't have another competent backup SF. If Sully went down, you at least have Wilcox and Green who can theoretically play PF (although there would be a loss in rebounding.)
That said, I think Sully is more "untradeable" than Green, just because he has the potential to turn into a better player. A legitimate starter power forward who rebounds and scores in the post would have more value than a mediocre backup (or even starting) small forward.