The answer is both.
For books you value, anyway.
I use a Kindle Paperwhite. I bought it two years ago for traveling, but I ended up using it at home quite a bit as well.
Everyone who reads a lot knows there’s nothing like seeing black ink printed on the white pages of real, physical book. It’s easy on the eyes, and draws you in to the world of what you’re reading.
Conversely, reading on your phone or iPad causes eyestrain and keeps your brain awake by flooding your eyes with blue light, which simulates daylight.
Kindle, on the other hand, uses a system of electrically charged capsules containing ink particles (as far as I understand), so you’re looking at real ink. It’s not a perfect substitute for paper, but it’s leagues superior to phones and tablets.
Another great feature is the dictionary and wiki at your fingertips. Come across a word you don’t understand (or think you understand, but aren’t sure)? Just select the word and a definition will pop up.
Much better than lugging a dictionary around for when you’re reading a challenging book like Sanction by Roman McClay.
The cost of a Kindle title is generally significantly cheaper than paperback, though you’d have to buy a ton of books before the device pays itself back.
Of course, there are downsides. It’s battery powered and needs periodic charging (though it keeps it much longer than an iPad, for example).
You’re also deprived of the publisher’s chosen page size, layout and typeface – all things that add to the experience of a book.
You have several typefaces you can choose from, but the convenient travel size of the device is smaller than a regular paperback (at least the Paperwhite is).
My advice: if you really like a book and want to keep one for posterity, buy it in both formats. Kindle for travel and easy learning, hard copy for the intended experience and the library collection you want to pass on to your children.