I was only joking around with you, I have no actual interest in arguing the superiority of the two powers.
In answer to your question, though, the forces of the Imperium are usually strong-willed by default, unless they have been tainted, as seen in the Traitor Legions. Commissars especially are bred for the sole purpose of shaking away any thoughts of heresy, and shrug off any petty mind magic. Veteran Commissars get plenty of practice when dealing with Chaos Sorcerers, who deal in such things.
Vader's pose needs to be a lot more aggressive. Maybe against a Space Marine chapter master that would hold water, but not a commissar.
I picture this crossover going something like this:
Palpatine: "You would destroy this Galactic Empire with only a thousand of your... Space Marines?" Chapter Master: "We would destroy your false Empire with only ONE Space Marine! You are superior in only one respect!" Palpatine: "And what is that?" Chapter Master: "You are better at DYING!"