Or is it just good ending bad ending etc with no real canon ending? If so I want to get the main canon ending first cause I was planning on taking a break from gaming after beating this game. So I want to finish the story with the canon ending if there is one.
the closest thing to a canon ending in terms of actual outcome is the neutral ending. don't align yourself with any reason, don't finish the optional dungeon until after you enter the final dungeon, and when you meet up with your teacher in yoyogi park after the obelisk, give compassionate, "human" answers to her questions (it's either yes to both or no to both, I can't remember which).
if you want to do everything there is to do in the game, though, finish the optional dungeon before starting the final dungeon. it's not exactly a warm fuzzy ending, but you get an extra final boss, another ability, and the chance to fuse some pretty nasty demons.
Digital Devil Saga/Avatar Tuner isn't related to Shin Megami Tensei, they're two separate stories. The Demi-Fiend popping up there was just a nod for fans. Plus, since that bonus fight is in the first DDS game... well..... I won't spoil DDS for anyone reading this but if you've played both games then I think it's a bit much to reason the Demi-Fiend going from Conception Tokyo to the Junkyard.
Live GT: HiroTsukasa Pokemon Diamond FC: 5069-0474-9960
I hope the TDE is canon, it'd open up some pretty cool possibilities for the conclusion of the SMT series. Of course, Atlus is interested in making money, so they'll probably pick the neutral ending, which doesn't rock the boat so much.
Persona 3 FES Intro: http://youtube.com/watch?v=oyV9jrMSnww The Kill Everyone Project: http://xhref.com/17a