It's not mental. Presumed we talk about elite footballers, there is no absolute quality regardless of the role a player plays. And there are certain setups in which Benzema will indeed be the better player than Agüero, because he can do things Agüero can't (and vice versa, of course). Read @berbatrick's post above, I think he's got it exactly right. Under Ancelotti, Benzema was fantastic in that false 9/CF hybrid role and no one in world football could have done that better.
On the other hand, Benzema's role has changed under Zidane. He is now playing more like a regular CF, scoring more, but not connecting B+C as good as before. So Agüero might actually be a better fit for that role. Then again, Real is operating with crosses a lot and while Benzema's heading ability is not great, he is certainly better than Agüero in that regard.
So it really depends on what you want from a striker.
