Williams is having a fine year. Both Nix and Williams are statistically very similar, and their teams are winning. Nix does have 4 more INTs, but my problem with Nix is some inaccuracy.
I don't think Nix is a headier QB than Daniels. Daniels makes much better reads and processes much faster than Nix. I think Nix has shown some problems when it comes to making the right reads.
As for Daniels and injuries, I'd say his leg injury was a freak play. Given the players we've seen at QB with slight frames, I don't know why we're projecting that he's going to be injured frequently. He also weighs 210 pounds (supposedly). It's on the light side, but plenty of QBs have weighed that much. He'll likely gain weight as the years go by as well.