Richardson may or may not be banged up right now. Mcfadden you have to believe will break out over next few games. If you are a keeper league though do the trade. Richardson and Jones have more value going forward. But Marshall lots of value in PPR, I think he finishes top 5 WR in that format. Wallace and Brown not as consistent in a PPR.
How are you looking for the playoffs? As in does it look like you will make them for sure. If so check the matchups last few weeks of the season. Which one has the better matchup is really a big factor.
Good luck. I have Marshal on my fantasy team and probably would not do the trade. My thinking being Richardson banged up, could get worst, McFadden yet to break out but IMO will. But then with Jones he could get sat at the end of the yr. Falcons 6-0 and could run away with division. If that happens players that get dinged often dont play as much, or get sat as a precaution. Lots of ways to get screwed.