I'm trying to decide which one would be best for graduate school to accompany my air. I have a MBA and would like to use the iPad to store my most of my textbooks to lighten the weight of my bag.
Thanks!
Your situation and mine are nearly identical. I've got an MBA and I'm studying a programme right now and wanted exactly what you're looking for, something to complement well the MBA (mine is the 11"). I used to have a full sized iPad and didn't want something that big to lug around, as it seemed contrary to the MBA's size and weight benefit.
My Kindle (which I love) just doesn't work with PDFs, so that was a non-starter really, what I wanted was something like the size of the Kindle but could display PDFs well enough to read (articles, books, magazines, scans of books, some of which are very old, and sometimes the scans aren't just one page, but two). I looked at other higher resolution options, but in the end went with the mini and I couldn't be happier. The screen, despite what some on here say, is *not* an issue. I can read a double page scan without zooming in. It's not ideal, and what I like to do is clip the page in Acrobat so there's just one page, but if you don't have that option, it's still readable. Also, there are tools like Good Reader where you can set a clip for every page, so you eliminate any extra white space on each page and it automatically maximises your viewing area.
The screen on the mini I find to be wonderful. I know it's not a retina display, and while I would like it to have one, it's far from being a deal breaker or rendering the device incapable of delivering lots of functionality. I play games on mine (which its great for), I do all the other things and more I did with my full sized iPad, but I love this one even more because the size and weight and thinness of this thing is something that makes it so great.
The iPad 4 would not be a bad decision, I would never say that, but if you really are looking to complement an MBA, I think the mini is that and more.