Well, the news just broke that Willis McGahee has a torn MCL and fractured lower leg and will be out anywhere between 4-8 weeks. Essentially ending his fantasy season. The Broncos will most likely use a committee with Ball and Hillman, but the upside grab is Hillman.
Ball is just a guy. Hillman has shown explosiveness in his limited time spelling McGahee, but may be hindered by his pass protection inability. It will hard to keep him off the field at this point, as he is the future RB of the team. I would pick him up in PPR leagues and feel good about him going forward as a flex play or RB2 in deeper leagues.