What are the advantages and disadvantages of iBooks and Kindle books for my iPad?

Even though I have had an iPad for a while, I haven't taken the plunge to buy any ebooks yet. I'd like to make a decision on whether to prefer buying iBooks or Kindle before I buy any so that I can keep my collection in one place. **iBooks or Kindle: What are the reasons to choose one or the other?** I also have an iPhone for some reading, and don't imagine I'll ever buy the Kindle hardware (as nice as they are at present).