This is a really interesting thread. I've used multiple screen sizes and types to read pretty much everything at this point. I've now settled on an iPad mini 4 and absolutely love it. I use it to read .epub books, textbooks, and PDFs in iBooks and so far, it's the best reading experience I've had. I actually prefer it to my older Kindle, since the clear retina screen feels way easier on my eyes than the not-quite-white against not-quite-black text that the Kindle had. I also like that the iPad's screen emits its own light. I know I'm way in the minority here, but I'm very finicky with my reading light and I get eyestrain pretty quickly if the ambient light hitting a printed page is even a bit too dim. Weird as it is, the backlight of the iPad mini 4 eliminates that issue for me completely and I get eyestrain way less than when I'm reading from a printed page. I know that the newer Kindles would probably eliminate that issue for me too, and I admit I haven't had a chance to try them yet, so it might not be a fair comparison. However, I'm definitely happy with my mini 4 and wouldn't change a thing.
I had a mini 2 before the 4, and the newer screen is just phenomenal in comparison. I always knew the mini 2's screens had issues, but I guess I just got used to it over the years. But now that I have a mini 4, there really is no comparison! The colors and clarity are just incredible. I used to use the 9.7" iPads before I had the mini 2, and I always thought they were a bit too big for book reading to me. As much as I prefer backlit displays for reading, the 9.7" was just
too big, emitting too much light, especially when I was reading in bed. The mini 4 is closer in size to your average paperback, so I feel most comfortable with a device that size.
As I mentioned, I use iBooks for pretty much everything - .epubs, scanned PDFs, textbooks, peer reviewed articles, magazine article scans. Even though the mini's screen is smaller than a printed A4 page, I have no problem zooming in to read the text of scanned pages, or just holding it horizontally, which makes the image about as wide as a printed page. When I was in law school I invested in a feed scanner and scanned all of my textbooks to read on my Mac and iPad, which was truly a lifesaver, as law school textbooks tend to be in the 1000 page range and I have a bad back.

It's a very simple setup but works perfectly for me. Considering all the scans I have, I probably have like 20gb worth of iBooks stuff on my iPad.