I am another user who actually had both and used both, so I'm not talking on speculation here. If you want to read books, like novels, without graphics, then the Kindle wins hands down. The screen is easier on the eyes (IF you are reading in good light; there is no backlight), the unit is much lighter so holding it is easier over long periods, and with all due reverence to the iPad user interface there is no way "flicking" to change pages is easier, quicker, or more intuitive than simply clicking a large button that is naturally right under your thumb. In addition, battery life is measured in weeks, not hours. I take week long cross country trips for my job and I never even bother to take the Kindle charger with me. And downloading via the Sprint cellular system, for no connection cost, means I can purchase new content anyplace, anytime.
As far as content goes the Amazon store has ibooks beat by a long shot, and there are tens of thousands of free public domain books that you can download without even going through Amazon.
Now, with that said, I recently sold my Kindle after getting the iPad. I'm not a big novel reader, and the kind of reading I do lost a lot without having the pictures and graphics. In addition I sold my netbook and I can use the iPad for browsing, reading, e-mail, etc. It's not a bad ebook reader, but nowhere near as good AT THIS ONE TASK than a Kindle.
So my advice is simple. If you just want to read ebooks get a Kindle, or one of the other dedicated ebook readers. If you want a multimedia device for more than just books, buy an iPad.
I hope this helps.