If we are talking about dynasty value in here also, is everyone so quick to discount Chris Johnson? I realize he has had a rough 3 games to begin the season, but I have yet to see 1 person bring his name up in terms of a dynasty player you would consider moving McFadden for. Is it possible he has fallen from grace that fast? If yes, I need to go out and start trying to acquire him in every league I am in- this is the same Chris Johnson that everyone said was a once in a decade talent that was destined for the hall of fame as recently as 2 months ago, right?. As for ranking players from a talent standpoint, I put it something like this
1. Adrian Peterson
2. Chris Johnson
3. LeSean McCoy (I was slow to come around to him, but am now sold)
4. Darren McFadden
5. Ray Rice
6. Arian Foster
I think it falls to another tier at that point. I truly believe there is that much of a gap between Adrian Peterson and every other running back in the NFL (and I have posts dating back 1 or 2 years ago that state the exact same thing, my opinion has not changed). Compared to McFadden directly, Peterson does basically everything McFadden does, only he does it better. They both possess roughly the same speed, but Peterson has better moves, more power, has proven more durable, has a better nose for the end zone, and seems like that rare breed of player that has the body to remain effective beyond the normal shelf life for running backs (although I admit that last part is strictly a gut feeling). About the only thing McFadden is better at is his receiving skills, but I don't believe he is significantly better and wouldn't be surprised to see Peterson come close to equaling him in that regard one season. I also think you can switch up any of the running backs from 2 through 6 and can't fault anyone for having them in any order.
To answer the question, I think you should instantly trade McFadden for Peterson if the opportunity arises, otherwise you are just shuffling cards between the other players. I am not a proponent of trading like players unless you get something in return, so I wouldn't trade, say, Ray Rice for McFadden just as I wouldn't trade McFadden for Ray Rice.