I've used all three of them, and from my experiences (and observing others), an iPad is near the opposite of an effective study tool. Writing documents is a pain, drawing on it is a bother, and it's annoying to get your PDF files on it. I've taken all three into my uni lectures, take me being on facebook on all devices as a constant, but the iPad doesn't facilitate for note taking, the Android one has a cool keyboard for notes (but can't draw on it easily), and the windows one is just as functional as my computer, so I can take notes, type stuff, and have the lecture slides on my computer.
Definitely recommend a Windows Device. If not, android ones aren't too bad as they have cheap extensions and apps to make them more user friendly.
Just as a general comparison of size (and moreso me showing off), here is a picture with the tablets, so as you can see, if you're going to carry an iPad around, a Windows Tablet is not that much more to add.
