Going to differ here. Also, going to plagiarize myself, since this question came up in another forum I belong to. I would go for the iBook, but I'm not you.
Honestly? Ask yourself if what you'll be using it for will justify the added expense. If you're a gamer or will be majoring in graphic arts, you will probably appriciate the greater graphics processing ability and higher screen resolution of the Powerbook. If you don't need that because you'll mainly be using it for word processing, web surfing and email like most students, especially if think you'll be using be using it wirelessly, get the iBook (the metal case significantly impairs the ability of the exact same AirPort card to get a signal -- a friend and I sat next to each other with her barely able to get a signal on her PBook while I was at three out of four lines on my iBook).
Otherwise, you have to ask yourself if a pound less weight and a minimal processor difference (1.5 Ghz PBook vs 1.2 Ghz iBook) justifies the added expense ($1499 - $1699 PBook vs $999 iBook). For me, I went with the more expensive 14 in iBook a couple years ago and it's been fine. I've just dropped in a 1 gb stick of RAM and that was only $130 (from Crucial, they're the absolute best, IMO) last week and only because 128 mb was not cutting it for Tiger. Have I seriously suffered for not having had the most powerful computer out there? Nah. It'd be nice to have a DVD burner, but I don't know if I'd actually ever use it. And now that I have the extra RAM, I can't really tell that much of a difference between my 1 Ghz iBook and my friend's 1.5 PBook with 512 RAM.
Like I said, the iBook works for my needs and I couldn't justify the added expense of the PBook for myself, but you know if you need the extra power.