My kids have a kindle and I have an iPad.
Doing it all again, I would stay with the iPad.
There is a Kindle app (for the iPad) that lets you store and read the books in the same manner as the kindle. The iPad is more intuitive. There are reader apps (free) that allow you to read .PDF, .DOC and .PAGES formatted documents.
The browser on the kindle is experimental and very limited.
Given a choice, my kids reach for the iPad every time.