Best Answer - Chosen by Voting
They are synonyms so, in most cases, you can also use either word.
Ex: "I have passion." "I have ardor."
"My ardor was increasing." or "My passion was increasing."
Ardor is more of the intensity of a emotion and passion is just a powerful emotion.
So, "The passion within his voice was noticeable." or "The ardor within his voice was noticeable."
I believe it would be rare for you to have to use the other word... If you do feel as if one word should be used instead of another put the definition in sentence and see which one seems less awkward.
Hope this helps^^
