If anything it's more of an academic vs layman divide. Academics generally use the pronunciation 'tin ni tus.'I like how Jack pronounces the word "tinnitus". This is how I choose to pronounce it. Is it the case that the majority of people pronounce it the way Hazel pronounces it? Or is it an American vs. British English thing?!
I've noticed that it's predominantly Americans who use the 'tin night tus pronunciation, but I've never liked this way of saying it. It's always seemed like a mispronunciation to me, and I feel it has come to be accepted because so many people use it.
That's just my opinion though and I know others don't like the other way of saying it. At the end of the day it's just a word, and in the grand scheme of things, I don't think it matters how one pronounces it!