Kindle vs Ipad: Kindle has a much longer battery life, is lighter, easier to read with and is cheaper (plus the free 3g).
Ipad is much better reading for pdfs because of the zoom.
If you are doing a lot of flipping through pages (and not just reading a book straight through, the ipad might have an advantage... I don't really do this except with pdfs). Also, the new Kindle is supposed to be a little faster.