I prefer kobo over kindle-you don't have to leave the app for iBook or kobo to shop.
I have not noticed any price difference between iBook, amazon or kobo-then again I have only shopped for a few titles.
Now-amazon and kobo have wider selection of books, I don't believe this will last for much longer.
I have the Kindle 2 and the iPad. I think it is easier to read books on the Kindle. The prices for books are the same for both. But the iPod definitely has more functionality. Right now I'll have to have both to enjoy each of their unique features. I plan to buy books on Amazon, and if I want I can read them on the iPod.