Over the weekend, I field-tested the iPad vs. paper book, and added a photo of a similar environment with Kindle (published and photographed by Len Edgerly). As can be seen, only the eReader/Kindle can be read outdoors in daylight, even in shade (click images for full size).
As you can easily see, the iPad is unreadable while the Kindle (and the regular book) are perfectly readable. So, if outdoor environment in daylight (or shade) is a requirement, the iPad (and presumably, other tablets) will find it challenging. Screen covers might help – but I haven’t tested those.