I was approached by O'Reilly very early. I wasn't even considering writing a book prior to that. However, my experience with O'Reilly has been _fantastic_. I was never writing the book for money, I was writing it for publicity and to help push the project forward. For these reasons, O'Reilly was the clear winner.
Additionally, everyone I've ever worked with at O'Reilly has been very professional, and they've been doing a great job helping push the book forward. They even rushed the production process (editing, indexing, proofreading, printing, etc.) from their typical timelines down to only a couple weeks so the book would be fully ready for VelocityConf in a couple weeks. And they did it! I'll be at VelocityConf signing books.