I honestly don't think that right now he's that much of an upgrade, if he is an upgrade, over Casspi. Casspi is a better 3 pt shooter, and rebounds just as well as Ariza. This is Casspi's second year in the league, and contrary to what was posted earlier, this is Ariza's 6th year in the league and he is 25 years old.
My point is, that I doubt Ariza is going to get much better than he is right now. Whereas Casspi still has upside left, and is as good as Ariza is right now. So why make the trade, unless your considering him as Casspi's backup in place of Green. I'd rather hang onto Greene, unless we can get a serious upgrade.