This is a real tough one.
Im gonna lean towards Brown for a couple of reasons.
Firstly im pretty confident that Brown will get the goal line carries, something im not so sure about Griffin yet.
Secondly I think Brown should be able to maintain his position as the primary back for the whole year, whereas Griffin going on Shanahans history maybe replaced by the rookie anytime during the season.
According to ESPNs Ease of Schedule rating, Griffin has THE easiest schedule of all RBs, Brown has the 5th easiest.