What’s the difference?

Reading is reading, as long as the brain is making meaning from visual language. What is the line between a billboard and an enormous outdoor LCD screen? A book and an iPad?

I think the level of comprehensibility and the transparency of the writing system are more important than the specific medium the text is presented in. A text with 1/25 unknowns (96% comprehensible by vocabulary) on a digital medium will be more effective at boosting acquisition than a text with 1/5 unknowns (80% comprehensible!) on paper, assuming both are clearly visble, use whitespace appropriately to help reading, etc.

In fact digital media have certain advantages, such as the ability to vary font size, which not only makes things easier to see but also changes the line splits in paragraphs, making a reading feel “fresh” the second or later time around. (I use this a lot in proofing books.)