Will sell considerably better on the PS4
Sony consoles have had better dark souls sales in the past with a much higher attack rate
The difference in install base between the Xbox One and PS4 is far greater than last gen were it was about equal
The Souls series are big in Japan where the Xbox One barely exist
Bloodborne may draw sales from Dark Souls 2 for people who can only afford one game in that time frame
Bloodborne will have already caused many fans of the souls series to purchase a PS4 who will then likely purchase Dark Souls 2 at some point.
My guess is that the first week for PS4 is three times higher than the Xbox One version at least.
This is the Game of Thrones
Where you either win
or you DIE