The way the draft seems to turn out (Brown and Williams gone at #8 ) I would do this trade. Brown and Williams have a bigger upside, since they seem to fit our offense better, but Henry is a proven player. I would still rate Brown/Williams over Henry, but I would rate Henry over Benson, since he doesn't have the same upside for us due to his running style.
It's true that our leverage will take a hit if we misses out on a RB in the draft, but what happens with Buffalo's leverage if the Phins, Bucs and Cards all get a RB on day one of the draft? It's very likely that these three teams all go for a RB, while it's very unlikely even one of these teams will pass on a RB on day 1. So, if this trade is going to happen it'll be before the draft begins IMO. The Bills are pressured to make it happen, because they're potentially left with no interest in Henry.
Trade for Henry and draft Sproles at #96 as 3rd down back and KR/PR. That should take care of the RB position.
Then we can focus CB, LB, G and TE in the draft.