What's easier to learn from: digitally printed text or physically printed text?

Studies have been done on fonts, showing that people read printed serif fonts 10% to 20% faster than sans serif fonts. The little serifs helped to distinguish the look of a letter from other letters, and it was easier for the visual system to distinguish. Back in the early days of the web, people used serif fonts. They looked pretty terrible on low resolution screens, so people developed fonts that were easier on the eyes, such as Ariel and Verdana. Only recently have screens gotten good enough that serif fonts might make sense again, but there is inertia, and it's going to take a while to go back. Most of the books I have on my old electronic paper Kindle, which I love much more than any fancy LCD screen, use serif fonts, and I find that a lot easier. The contrast is good, and it works under most lighting conditions. Right now, though, I think this means that paper is generally better than screens, with higher reading speeds that can be translated to better comprehension at lower speeds. This will probably change in a few years.

There are some good answers here. I'll make one point that I think is not widely appreciated.  Publishers ignore most of what they know about making print books easy to read.  The parameters of readable print are well understood: lines of 60-75 characters, 11 or 12 point type, open leading, generous margins, trim size in proportion.  For a variety of reasons, mostly financial, publishers routinely abuse their readers by compromising these qualities with small type, tight leading, narrow margins, unwieldy trim size, doorstop length.  All these factors tend to undercut the advantages that are, theoretically, available to print. Conversely, ebook readers follow the well understood principles of making text easy to read: 7 or 9" diagonal size, light weight, thin binding, ample margins, type resizable to optimum.

At least in terms of my laptop or desktop, I would rather read a physical book.  The only exception might be if I got extra content for the digital book.  I think I can scan more quickly in book form.  I also like the ability to easily mark up a book. I can also draw directly in a book. The main advantage of digital texts is: 1. searchability/findability 2. being able to carry multiples (ie a full bookshelf) 3. cost is usually about 15 to 20% off

One benefit an e-reader has over physical books is that you don't have to consult a dictionary for vocabulary you don't understand too well or have familiarized. This decreases reading time marginally but increases your reading comprehension and of course your vocabulary.

Good question.  I will look forward to responses.  I am taking online classes...I think it's a bit tedious trying to read a textbook in pdf form.  I do like the ability to highlight, cut, and paste.  However, somehow I feel I'd retain more information if I had the physical textbook in hand to highlight, mark-up, and label with post-its.

I have faced similar problems. But these days with the advent of larger screens and higher resolution in mobile phones and tablets, it is much better than reading text from a laptop screen or a desktop screen. I have tried reading books and comics from tablets, and I found these gadgets to be very handy with a lot of options and features like brightness adjustment, invert colors.