Assuming that I am playing to win now and need a RB2, I would deal Juju for Freeman, since JuJu's upside is capped as long as Brown is in PIT.
I don't see why Freeman can't give you 3 more solid years.....he doesn't have that much mileage, he shares the workload with Coleman. In dynasty, I generally don't look more than 3 years down the road. A lot can change in 3 years. Questions remain in PIT.....you don't know when Ben is going to retire, or how long Brown stays in PIT, or if JuJu can elevate his game to be an elite WR. Right now Freeman is a strong RB2. If Coleman gets hurt, Freeman can produce elite numbers....like he did in 2015. I would rather take the guy that I know can produce and still has upside....which is Freeman.
Now if I am rebuilding, then I would lean towards JuJu, since I would be wasting Freeman's good years when I am not a contender.