I heard something along the lines that Demon's Souls is more linear while Dark Souls and Bloodborne have a more Metroidvania style.
Story wise Demon's Souls and Bloodborne interest me more.
Yeah Demon Souls is definitely the more linear one. Unlike all the other games it doesn't have a seamless interconnected world. You basically get to a hub area and from there you can be transported to different locations/levels.
As for the Metroidvania style, they do have some common elements like the interconnected worlds, usually with a boss at the end, as well as shortcuts you can find to make traversal faster, but I wouldn't call any of the souls game a "Metroidvania". As I said, there's a lot less focus on backtracking and there also aren't big changes to how traversal works. To me a big part of Metroidvanias is getting new skills and then going back to earlier areas and using those skills to reach new locations, and the souls games don't have a lot of that. You might occasionally find a key or some item which lets you enter a previously inaccessible area and there's more central locations you'll probably get back to multiple times. But it's not like you learn to double jump and then have to go back to earlier areas to find places to reach with your new double jump. Your core skills are almost the same at the beginning and the end of the game.
If anything, you might find areas which are really hard, so you avoid them and go in some other direction and then come back hours later with increased stats and better gear to make things easier.