Both similar but ibooks just has the edge for me. I think I'm correct but Kindle doesn't appear to have a dictionary function on individual words like ibooks. Not a huge problem but it is a nice addition.
Both offer free "classic" books but if you do look at the books on offer in Amazon you'll find they offer free current books nearly every other day. I downloaded "No Mercy by John Gilstrap" and "The Heir by Paul Robertson" for free - no idea what they are like!
Also noticed ibooks don't offer the same volume of current books (unless I'm doing something wrong!)