I prefer PalmOS to WinCE, er, PocketPC. If going the Palm route, go for PalmOS 5.x (better multimedia), a color screen, and as much RAM as you can afford. An expansion slot (typically SD) is good as you can store MP3s or use it for all kinds of other things.
If you're looking at a unit to help you study, though, the real determining factor will be the apps you'll want to use. Which platform are they available for? For decent listings of apps, go to handango.com and palmgear.com. Even download.com (or whatever the cnet site is called nowadays) has an ok selection.
BTW, PDFs need to be converted for the Adobe Acrobat Reader for Palm. The prog, free as all Readers are, has a Windows/Mac client to do the conversion on your PC. Then you sync the PDF to the Palm. This is Adobe's fault and it's a sucky limitation.
