It is always helpful to give the whole picture in the opening post.
1. League type and size
2. Scoring – You mentioned PPR, but Passing TDs and bonuses are helpful
3. Starting requirements including flex positions
4. Full roster.
You probably wouldnt start JD5 or Dak over Lamar honestly, so thats just an equal bench move to me, but I also like Mostert more than Dowdle. I may do it too. Without Tua, Miami may run more and I always prefer a handcuff RB to a random backup