A picture says more than 1000 words, doesn't it?
While Kindle is unique with its mobile connectivity sans the cellphone plan, that feature is rather irrelevant whenever there is Wi-Fi available for iPad. Beyond that, it is difficult to see why would anyone want to read their books on a device with a small monochrome screen with a lame keyboard, no touch capabilities and no other use.
iPad provides clearly a more engaging experience with a larger, sharp, color screen, touch capabilities, and lots of other applications available. iPad is of course more expensive and it is also a bit more heavy for "holding the book in one hand". On that front. the iPhone/Touch with a book reader (e.g. Kindle reader) is still unbeatable.
I was lucky to have them both at home over the weekend.