I'm planning to play the game first time and I have heard that the game has two different endings and I was wondering does the extra chapter consider one ending canonical or if it doesn't matter storywise which ending I get in main game? So, my question is to which ending I should aim to get the best narrative without continuity errors?

I would appreciate an answer without any spoilers.

the ending you get with the "extra chapter" gives you more explanation about the story of the original game, is after the first game and it doesnt happen in the same time neither

Thanks for the information.