Which I should I get?  I barely get enough and I really am ecstatic for KH3.  I want to get a PS4 but at the same time I've seen that the console itself is pretty expensive.  I also have to move in April.  So, I wonder if an XBOX One would meet my costs.  I would like to get the preorder but again, I don't think I can afford it.  What would you recommend and which is the better deal, XBOX One or a PS4?

The choice is irrelevant unless you haven't played the previous games in which case I would go for ps4 with the "all in one" package on the psstore.
If you have a 4k screen, xbox one or ps4 pro, otherwise it doesn't matter.

I bought a PS4 Slim for Kingdom Hearts 3. It's less expensive then the Pro so if you're looking to save money I'd go for a PS4 Slim. I'm also not a hardcore gamer so that also played a factor in going for the Slim version. I don't know anything about the Xbox though. Actually I'm probably not that qualified to give advice on this sort of thing.

I'd say get whichever is cheaper at the store/on sale. As cyntix said, If you've played all the previous games/know the story it doesn't matter if you get it on XB1

As far as I know, slims perform the same as original PS4/XB1. If you want KH3 at max FPS, then go for PS4 Pro or XB1X. I'm sure the only real difference will be FPS. Even then a regular PS4 or XB1 should be fine. I also haven't seen much of a difference in real life when it comes to it being "Best HD". That will also depend on if your tv is HD(Regular 1080p is fine).