Just based on Calzada's film I just looked through from 2 or 3 games this year, he has chance to be a on par with Nix in regards to the passing game, maybe even better in some areas. Bo showed some ability this year to stand in the pocket and deliver a throw under pressure, but Calzada did that on a much more consistent basis in the film I saw. Bo bailed way too much, and operated out of the system/progressions way too often. Both have similar arm strength, but Nix has quicker release and can throw it better from different angles. Calzada may be better at the deep ball, but I honestly didn't see A&M take many shots on film.
Also, regarding Calzada's "happy feet" that some have mentioned, I think a lot of that is just from his in-the-pocket technique of consistently moving the feet to stay in throwing position, sort of like Peyton Manning always did. Now of course he does bail some when he doesn't need to, but I think that part of it may be overstated.
One last thing - Calzada looked terrible at times carrying out fakes in the run game. Very lazy movements.