My initial reaction was Henderson by a wide gap, but (at least for 2017) perhaps the gap is not so wide.
Henderson may have a role in Denver's offense even if both Anderson and Booker are healthy. Henderson's role as a speed back could tumble, however, if Charles proves he has something left to offer. In any event, I think it would take at least a CJ Anderson injury for Henderson to become fantasy relevant as anything more than a very deep bye week cover, and even then, he may share time with Booker. It may take injuries to both Anderson and Booker (and Charles not returning to form) for Henderson to have any significant impact for this season. As a dynasty stash, however, I am all in on Henderson.
Like Henderson, Carson has a lot of bodies to leapfrog, and, despite how good he has looked so far in the preseason, I have my doubts Carson is going to usurp the power back role from both Rawls and Lacy. Carson may need an injury to at least one of those two to have any role, and injuries to both to have a fantasy relevant role. Prosise would continue to be a factor, further limiting any value. I am not quite as bullish on Carson as a dynasty stash, but that is not to say he doesn't have some dynasty intrigue.
Both Henderson and Carson find themselves currently buried behind stables of talented but oft-injured backs, but we have seen how quickly that can change, especially at the RB position. If I am grabbing just one (assuming equal investment), I want Henderson. If either of them is going to become a season-changer, Henderson seems the more likely. He has the skillset to become an every-down back, and he runs behind a superior O-line.