Of course, you don't need a device to read books purchased from either store.

I'm not sure what you meant here. You certainly do need a device - worst case, your PC. Was it your intention to say that you didn't need a mobile device - Kindle or other?

(For me reading on a PC is not an optimal use of that time - I'd rather be writing or otherwise producing when at my desk, whereas reading in places where I can't write - on the tube, in the loo, on vacation, and so on - has always felt like a win. So far, I haven't found anything that beat physical books for that purpose, though I've recently decided to give e-reading a try.)