Well if he can fix his throwing issues he'll always be more valuable at third than at first.
The question what gives you the MOST value overall? Zimmerman at 1st with Rendon at 3rd? Other way around? Rendon on 2nd, Zimm on 3rd and someone else on first?
I'm stubborn and sticking with Danny at 2nd. I think Zimmerman will profile as a better 1B than Rendon. I think best value is Zimmerman moving to first.