Inconsistencies

[Inziladun]

Originally Posted by PrinceOfTheHalflings

As has already been noted, Saruman was not dominated. Don't be confused by the movie-Saruman. Two more things - Saruman had a palantir far longer than Denethor and Saruman was being corrupted by his lust for the Ring. However, despite being entitled to use his palantir it seems clear that Denethor's spirit was broken by Sauron ...

Saruman was under Sauron's domination, in that he no longer opposed Sauron's victory, and was in fact supporting it, assuming he could not get the Ring for himself.
Of Saruman and Denethor, it was apparently Denethor who actually made first use of the Stone available to him.

....it is probable, considering Denethor and what is said about him, that he began to use the Anor-stone many years before 3019, and earlier than Saruman ventured or thought it useful to use the Stone of Orthanc.

UT The Palantíri

Also, though the selected visions Sauron fed to Denethor, and the strain of using the Stone itself were certainly factors in Denethor's general grimness, it wasn't until the seeming death of Faramir that he was finally mentally overthrown.