If you want the exact same typesetting as the physical book, pdf is ideal. On a big enough iPad, this would be readable like a physical book. But, yeah, painful as hell on a smaller screen.
I guess that must be other devices. I honestly don’t notice on the Nook. Maybe it was designed for it. Shrug
I do both, actually. But I don’t notice the difference personally.
There's not much of one until you need to reflow the book for a different reader