I like the AMOLED screen of my smartphone, and I use gray text on black background. With amoled, only the pixels that aren't black actually emit light, so it's quite easy on the eyes as there's very little light coming from gray text. At the same time it's readable even under low light conditions.
E-Readers might not have a bright light in the screen itself (the traditional LCD problem) but if you need a very bright reading lamp in order to be able to see anything (esp on the older generations with low contrast), it's not that much of a difference.
Still I wouldn't go for this if I didn't have the Smartphone anyway, as the phone was about five times the cost of a cheap ereader.
Apart from the display issues, many readers still have shortcomings when it comes to the software. Which might not be a problem since 99% of the time all you do is press the "next page" button, but it's still nice to have an extremely well thought out and responsive interface on the smartphone...