Reading on a glossy glass LED backlit LCD is sometimes difficult for some people - they report eye strain and headaches and such. If you are like that, then for a dedicated ereader, that pretty much rules out any tablet with a glass LED lit screen (iPad, iPad mini, Kindle Fire, Nook tablet, Kobo Arc, and so on).
If you do not like using glass LED/LCD screens for long periods, then a B&W eINK screen might be much better for you - like an original Amazon Kindle model, or a Nook Simple Touch or a Kobo Touch.
As long as the screen is alright to your eyes, and if you want to read magazines or enhanced eBooks with color art and such, then you would want an LCD screen, the choice between iPad or iPad mini is mainly a choice about the size and weight of the device. One nice thing about an iPad is reading in landscape mode on the big screen as that just works well with certain formats or book layouts (and is much better for magazines, IMO). However, the iPad mini is much smaller, lighter, more comfortable to hold for long periods and more easily slipped into a coat pocket or bag.
While I love my iPad, I actually use my Nook Simple Touch (the original model) if sitting down to read for a long time. It is the lightest ereader I've ever held, and the size is perfect for one hand holding while reading. The eINK is superb for text only novels and such, and it is a non-glare glass screen (it does not use capacitive touch for control - the touch control is via infrared LEDs mounted along two sides of the device, forming an infrared grid over the screen). My Nook Simple touch is about 1/3rd or less the size of an iPad weighs right about 7.5oz, while an iPad is a bit over 23oz. That difference may not seem like much, until you've been sitting holding it for a half hour trying to read. The iPad mini is only about 11oz, so maybe a nice compromize weight, as long as you like the screen for long periods of reading.
P.S. I do read a lot on my iPad too, as glossy LCD screens do not bother me, but if settiling down on a weekend to read a novel, I'll tend to pick up the Nook ST instead.
That also begs the question of whether you want a truly (or mainly) dedicated ereader, or do you also need tablet functions as well (email, web browsing, other apps/functions). eINK readers are pretty much ereaders-only, as the screen technology is really only good for displaying text, and is much slower to refresh than an active matrix LCD is.