I definitely prefer iBooks. I think the interface is better and I like the fact that you can share books with others on your account. AFAIK, you can lend your books with Kindle, but you can't read them simultaneously. The upside of Kindle is they seem to have a wider selection and sometimes the prices are lower.
There are free books available on each. I suggest trying a book in each app and seeing which you like better. Both Apple and Amazon have a Top Free Books section in their respective stores. Have fun! I love reading on my iPad!