Right now I have five screens in front of me - 4 LCD and one Eink - and there's text on all of them. It certainly doesn't feel like I'm doing different things when I'm reading from different screens.
I could understand the idea that there could be a major difference between reading from a CRT with a 60Hz refresh on one hand and EInk on the other - but the OP seems to be thinking we should link both together and put paper on the other side.
I think the biggest differences would come from reading straight text vs. reading documents with lots of hyperlinks and banner ads on the side etc. In that case there's a real difference because there are more choices to make - but it's what's being read not the medium.