I would take Harvin if you get any points for WR's getting rushes (our league awards 3 points for every rush a WR makes....since it is uncommon but Harvin gets rushes and especially if AP goes down)
If not I probably roll with Harvin still since he is the #1 WR on the Vikes but C.Henry intrigues me as well and may be valuable if he can stay out of trouble and healthy this year!
(On a side note I would pick up Cassell since you don't have any backup QB's but his latest injury has me worried and you don't have any backup WR's either so.....
So......I would pick up Cassell or Harvin (depending on who else is available at those positions) and how many rounds you have left? I think you could grab Henry close to the last round still so I would wait a little longer on him.
Standard H2H 12 Team 1 Pt PPR, Start 1QB, 2RB, 2WR, 1 FLEX (RB/WR/TE), 1TE, 1K, 1 DEF