It would certainly come down to either Breida or Mostert given that you have an abundance of WR options and only 3 RBs. Mostert looked really good against the Ravens and has looked good any time he has been given opportunities but with Breida likely returning this week it is hard to imagine he consistently gets enough reps to be relevant. Brieda is likely the way to go unless the 49ers coaching staff finally looks at his ypc and realizes they need to use Mostert more, not a bet I'd want to make.