If Amazon is an indicator, I think that R&C probably ate QB alive :P

I think Ratchet and Clank will outsell Quantum Break by quite a fair bit.

I have no daubt DS3 will out sell its predecessors but it will hit a wall and stop after the Hardcore fans buy it. QB and RnC will continue to sell thoughtout the months. Wont be many but will still get attention. Also RnC will outsell QB on many fronts, one being sold on a bigger install base and the other being sold to the general audience afterall its a kids game compared to an adult game.

That's a narrow way to view it.

Souls series certainly struggle to achieve more than two million, but it's a feat they've repeatedly done over and over again. Sure, they're not Mario or COD. Neither is Ratchet & Clank or Quantum Break, for the same matter. However, in Ratchet & Clank case, their sales went down and down with each new entry; you could say this new R&C is being heavily anticipated, but I certainly doubt it will do more than Up your Arsenal lifetime. So we're seeing two franchises - one which consistently sells more than two millions - pit against each other the same day. It will definitively eat some of its sales, that's for sure.

And Dark Souls stopped being niche ever since Demon's Souls, which sold two million, if we count digital sales. Heck, the Souls franchise have never been niche. There's a huge market that consumes these games, as a lot of games wished they could sell two millions per entry. Likewise, there's no evidence whatsoever that people who consume Souls games aren't interested in RnC; if anything, some users of this site are aiming to get both RnC and Dark Souls 3, so it's the opposite of what you suggest.

The last R&C sold 0.67 million. It's not exactly a huge franchise. (But I wish it was)

A game that sell 2M is niche? No Way. If QB sell 2M I will consider it a success (and I wish it will).

I think DS3 is gonna win, between R6C and QB I really Don't Know. R&C is not a super successful saga, but is like the first AAA platformer for ps4, and I think a lot of people will buy it.

QB is a highly anticipated game, but I think it will have legs more than a big launch week.

This new R&C is going to beat that by alot I suspect.

Quantum Break. I haven't seen merketing for R&C and QB is MS new big franchise. R&C sales will be low initially, but I can see them picking up a bit when the movie releases.

Certainly, but people are using the "R&C is an established IP" reason. That's like saying R&C will outsell The Division, cause R&C is an established IP while The Division is a new IP. It's just not how it works. 

Might be a tad soon to say, but...what a difference a generation makes ;)