It's pretty different, since I believe some things are locked off to you when you kill a monster.
But there are a lot of secrets, some things randomly happen on some runs and not others, dialog changes, and I almost want to say that the ending of pacifist is too good to just watch. And some stuff before that. So there's definitely reason to go back, especially since, the second time around, the game's a lot shorter.
Hey, hate to be this jerk, but I'm gonna hold off on gifting AS. It'll still happen, but I kinda wanna get some things and don't really have too much to throw around. Sorry about this, I'll keep an eye out, though, since the sale price is pretty this, currently.