I prefer iBooks, but Kindle seems cheaper and has more titles. Just curious to hear which you use and why. I think I'll likely have some books for each, but will lean toward iBooks.
Entirely Kindle. At the moment, the selection in the Kindle store dwarfs what's available in the iBooks store. That's the deciding factor for me. It's also nice to know that Amazon is not tied to any one platform. My books are still available in the unlikely and unthinkable event that I happen to pick up an Android or WebOS tablet at some point in the distant future.
I voted other. What I do is buy from Amazon (kindle), then remove the DRM and convert to ePub, and read in iBooks.
My local library (and it's quite common now) lends eBooks in the ePub format so I mostly use iBook or Stanza. That said I've never bought and eBook from the iBook store or Kindle Store.