I read on both, and kind of like reading on the Kindle better. It's mainly because I can read on eInk with less interruptions. On my phone I'm typically getting, text, emails, and phone calls.
I also tend to read outside during my lunch break. Where the eInk is a clear winner. At night if I can't go to sleep I'll read on my phone since it's much easier to read a backlit device in the dark than using a light source.