Silmarillion - Chapter 24 - Of the Voyage of Eärendil and the War of Wrath

[Faramir Jones]

Originally Posted by Inziladun

Indeed. Giving second chances was apparently hardwired into the Valar. Remember too that Gandalf was ready to pardon and allow Saruman to repent, though in that case an imperfect and merely temporary atonement would seem to have carried much more risk than a backsliding Sauron.

As you correctly pointed out, Saruman had only gone to the bad for a short time, while Sauron had behaved that way for millennia. By the time of the War of the Ring, the latter was so known for deception that he was called by Gandalf 'Base Master of Treachery'.