Oh brother. Seriously, if the main reason you are wanting a PSP is just to watch Advent Children, don’t even get either of the systems. Just buy the movie when it comes out on DVD later this year.
So from your explanation, I’ve told you that you can save money by getting the DS, since it seems the only reason you want to invest in Sony is to pick up that movie. And since it is a DVD release, you have to reason to get the PSP, according to your standards. So you have no choice but to pick up a DS now.
Better yet, smack yourself in the face for wanting to see it in the first place.