All things being equal, I guess Deng is currently a slightly better player, but given the circumstances: contract amount, contract length, durability, and age (I agree with the comment that he seems 'old' for his age), it doesn't seem like a worthwhile trade off.
This maybe one of those 'grass is always greener' type situations. I still think of Green as peaking and is as athletic as he has ever been, I guess I don't feel the same way about Deng (although I might be wrong). Also, Deng is known for his defense, but I think Green is quite good in this department and can play multiple positions.
Deng was once thought of as a future [super]star, but he never really reached his potential. I think the thought of Deng is better than the actual player we would be getting.