I usually just buy which ever is cheaper. I do prefer to read on my Kindle to physical books though.
Just bought a paperwhite last week and I love it. I've used the overdrive and Kindle apps on my phone, a Kindle fire, and my iPad mini and like the paperwhite much more.No, I read books to avoid looking into a screen.
Although readers with e-inks might be worth looking into.
I'm just starting too for environmental reasons (and my bookshelf is full and and I don't want to buy another book shelf)