Regardless, even if Nicodemus and Mab are indeed both defending reality, I'd say that there is a significant difference between their motivations behind doing so. It's Mab's responsibility to do so; Nic and the Fallen want to do so because nobody but them is allowed to break their toys—at least, that's my reading of it. There's a quality of possessiveness that Nic has that Mab doesn't, in my opinion.
I could see Harry becoming like Mab, but I could never see him becoming like Nic, despite Harry's inner monologue insisting otherwise.
Aside from all this, consider that Nic is still a mortal. He's got a Fallen Angel in his brain, but he's still a guy. That means he's constantly choosing to do the things he does, in my opinion.