Thanks for the help with mine!
First of all, given your starting WRs and RBs (w/out knowing the size of your league), I'd say you're in pretty good shape either way. You are lucky in that Rice and VJax are both outperforming their ADP. So your flex position is icing on the cake. I'd go with Charles for a couple reasons: Charles is going to touch the ball a lot, or so it seems. He'll probably get 4X the touches that Johnson gets. Johnson is one false move away from sitting again....Also, he's listed as questionable, and while it looks as if he'll play, if he doesn't he plays in the 4:00 game, which could give you trouble if he's declared inactive, depending on the rest of your roster. Jamaal is safer here. Good luck! (I don't think you'll need it).