See, I feel the exact opposite. Reading on the Kindle isn't comfortable to me. The contrast ratio isn't high enough and browsing around in a book is a bear with how slow the interface is. The mini is the perfect reading device to me.
I feel the same way. The older Kindles are basically dark grey on light grey instead of black and white, which gives me a headache because of the low contrast. I haven't tried the newer Paperwhite model, though. But even if the Paperwhite solves the contrast problem, iPads do so much more. Half of the reading I do is news articles, and for that, iPads give you access to full websites, while the Kindle -- well, I haven't tried it on the latest models, but last time I tried, the web browser was just atrocious!
Basis for your 4-9 hour quote? If that were true, I and a ton of other people I know who stare at computer screens all day for a living, would be screwed.
The mini is awesome for reading. Nothing else to say about it.
You'd that generally stare at it non stop. You get up to get a drink of water, to pee, to eat lunch. To look away and check out the new girl etc.
That's really the trick, stop every 30-minutes or so and do something else. Even just stare at the wall. That and the brightness trick will reduce eye strain