I'd recommend Fitz.
That way you can let Aguirre gestate in your mind for a while afterward

This ^
Especially if you plan to watch them in one sitting or relatively close together. Amazing films.
Bentley is a walking, talking premature ejaculation of desperation & stupidity.

'Fitzcarraldo' is the more pleasant movie to digest. I believe that for a typical viewer, it would have slightly more emotional staying-power; it will linger in memory longer. Especially so if you add to it 'Burden of Dreams' which recounts the making of the production.
The thing which sticks in the mind re: Aguirre is perhaps only the last scene in the film where the character simply won't back down.
So my recommendation is to view 'Aguirre' first and then allow 'Fitzcarraldo' to wash any unpleasant aftertaste from your palate.
Paul P. Powell, Pool Player