It's pretty clear that if you read enough, that an e-reader has to win.
Or maybe not ... ?
Let me be more careful. If the amount of CO2 produced by generating the electricity used in the distribution and consumption (powering the reader) of an e-book is smaller than the the amount of CO2 released in cutting down trees, making paper, manufacturing the p-book, and shipping the p-book then, for a sufficiently large number of books, e-books are more environmentally friendly.
Eventually the fixed environmental costs of manufacturing the reader become irrelevant.
Ergo, read more!
Counter argument: What if you read only second hand-books? A book can last a long time, perhaps longer than a reader ...