Witcher 3 is very long, and the dlc's make it even longer. If you're looking to kill a lot of time in a nice RPG, I'd go with Witcher 3. If you enjoy more of a challenge and more 'interactive' combat and replayability, DS3 is good. It's probably the easiest souls game, and it's a lot faster paced than One or Two. Both are good and pretty games.
Depends on your priorities. PS4 if you like playing a lot of games and have good amount of time to kill at home most times. Switch if you go out a lot, have a good amount of irl friends, or like indie-like games. There's no reason to get a Switch if you're looking to play something like Skyrim, or Doom.