My preference is Kobo, actually, but all dedicated ereaders have the same inherent advantages.
Firstly, e-ink is MUCH more pleasant to read text off of for hours at a time. An iPad with its backlit LCD display is like reading off a computer screen, which most people will agree is not great to do you hours on end. E-ink is basically just like looking at paper. Battery life is way better since e-ink displays also use no power to keep an image on screen, and they require very little processing power. You can use a Kindle/Kobo every day for weeks or even months before it needs to be recharged. An iPad will be dead in a couple of days if you read frequently. Most ereaders are also more portable, even when compared to an iPad Mini, but the displays are still large enough to be comparable to a paperback book. Basically it all comes down to the fact that ereaders are made specifically for one task, reading text. iPads and other tablets are obviously much more versatile, but they can't beat a Kindle or Kobo in the one task that they were designed for. It's just like how iPads or iPhones make really good cameras that will be good enough for most people, but they can't beat dedicated cameras. But these days most people just want one do-it-all device, so they will use an iPad as an ereader. Not me though. I'll gladly use my Kobo Aura for reading books and iPad Mini 4 for everything else.
