There's no comparison, really. Because they are two radically different devices. One is an ereader... period. And the other is a multi-app/multi-function tablet computer that happens to have apps available for reading ebooks.
I don't have an iPad (I'd get one in a heart-beat if I could afford one). But the Kindle would still be my main reading device... because my old eyes just wouldn't be able to survive the 3+ hours a day beating they would take reading on the iPad's backlit LCD screen. But I could sure come up with a lot of other uses for it.
