Dorian was born with a silver spoon in his mouth, and can be somewhat naive and romanticize things. Fenris was a slave who was tortured, physically and mentally abused, and probably sexually assaulted (it was hinted at, and I recall David Gaider had confirmed it at one point, but I can't locate where I read it- I am in the camp that it's canon that he was). Neither one of those sides cancels out the other, and Dorian glossing it over does not invalidate Fenris' experiences as a slave. If I had to choose, would I want to be a slave to Dorian or Denarius? I'd pick Dorian, I wouldn't need to ask any more questions beyond those two choices. If I had to choose between any form of slavery, even for a nice master who gives me good food and fancy clothes and treats me well, and freedom- freedom is the answer. Most people, I imagine, would rather be free (there are exceptions, there always are).
Dorian and Fenris are coming from total opposites of the spectrum. Both of their experiences are valid. One does not make the other untrue. Dorian is very naive in many respects, however, and Fenris is very bigoted against mages. I second seeing them both as companions to the PC, their arguments would be epic.
As an aside, David Gaider wrote Fenris and Dorian. I am confident that he is capable of remembering what he wrote for a previous character and being able to write another with an opposing view without it making the previous one invalid. Most authors will do this- they will create a precedent with one character and establish "facts" and lore through that character's eyes, and then deconstruct it with another character. It's a very fun thing to do and helps round out the fictional world.