The iPad has way more screen real estate than any currently available kindle or nook. Also, the e-ink kindles don't handle pdfs well. Before my iPad, I tried the tabs/songsheets on an e-ink kindle, and VERY quickly went back to paper. But with my iPad, I use GoodReader, a pdf/word/txt reader app which allows mark-up, folder organization, etc. I have all of my song sheets and pdfs of tabs, loaded and it's fantastic. It will download and sync with a dropbox folder, too. It can do 2 pages at a time, as well -- though that's in landscape view, so it's smaller, but I find still readable on a music stand.
At my local club, people are always impressed with my setup -- it's much easier than lugging around our whole songbook and adding pages to it, etc. And much more environmentally friendly.