I use both (and Kindle books sync your position between devices). It's nice to read on your phone because it is always there. But it is just more comfortable to read on a e-ink screen both because of the size and the fact that things like sunlight don't wash out the screen.

With a black background and Yellow or off-white text I have less problems. I also have a night text that is orange-ish or brownish if it's really dark where I'm reading.