I had Greene for the last 2 years. I also happen to have season tix. I can tell you that LT is still going to vulture touches, EXACTLY when you need them from Greene. I think that Greene will be a beast next year, but for this year, it will be RBBC. LT is still a good thread for the short dump pass (meaning that Greene won't get you the PPR points), and with the likes of LT and Keller in the mix, I still see Greene getting limited touches. Also, Plax adds something very new to the offense. Plax makes quarterback mistakes go away simply because of his size and talent. A throw that would have been missed by Edwards last year will be caught this year, so will lead to a few more passes and fewer rushes.
So that being said, I would do the trade for Mendenhall/DJax in a heartbeat.