Uhhhh.. I mean, this is a really interesting discussion, and I agree that there is no right or wrong. How could there be! No one decides/ has written the laws of collabing so as a result there can be no correct way to go about it. With that being said, I think that I would rather collab with people outside of my niche. Why?
Because if I were to collab with someone outside of my niche, I feel that it would be a more boring experience than the former, and I would learn more about that person/groups niche.
Take myself, for example. I would say I am predominantly a vlogger, but I have one or two other videos that reach out to different niches. I would love to make a video with someone who does, say, gaming videos or life tips etc. I think that I would broaden my horizons in terms of:
a) What I find interesting/ consolidate what I know I don't like
b) Give my (few) subscribers different content to keep them interested (AND if they don't like it, they would tell me/I would see a drop in viewer engagement etc.) and;
c) Give my channel / person more exposure in the YouTube community. It would be pretty boring (dare I say pointless) to just be known by a specific brand of people, even if they are your target demographic. The way I see it, Youtube is better when you try new things, meet new people and explore different aspects of your videos and personality that you never dreamed you would be able to discover