What's wrong with the 12" display if you don't mind me asking?
I often take my 12" to uni with admittedly very little in the way of books and it's easy to carry around, I think an extra 500g wouldn't make too much of a difference.
It's tough as nails too, I don't have a sleeve or anything like that, just a little padded section in the backpack and my iBook's been from Vancouver, throughout the Rockies, across Canada to Halifax, from Boston through New York and LA back to Sydney all in the backpack and it's running like a little champion - just a few "cosmetic" (read annoying but not unforeseen) scratches on the case.
You'll get better battery life out of the 14" but in terms of the display - I think it looks a bit fuzzy compared to the 12" due to the size of the pixels. It's a physically bigger display but the res is the same so you don't get any extra information on the screen. My 12" looks really sharp but damn I'd love a 15" PowerBook (don't need it of course but hey? you need goals)