Coffee lover, e-reader, and Instapaper developer Marco Arment put together what he describes as an informal survey of the current library landscape, including iPad and iBooks, Amazon's Kindle and Kindle app, the Nook color, and Kobo.
If you’re going to primarily read periodicals, get the iPad. If you’re going to read books, all of these platforms look like safe options.
Check out his comparison charts for books, magazines, and newspapers. It's not so much his results, which he cautions you take with a grain of salt, but the method he recommends for finding which device or app ecosystem is best for you that's compelling.