Voice Dream Reader is also very good on the iPhone. It can use any of the apple voices (download high quality versions in accessibility settings), and has other voices available. It can OCR pdf files that are otherwise inaccessible. It also makes a great audio book player.
More to the point, I’ve had trouble using Siri to speak in certain titles in iBooks on iOS. It would drop words or entire sentences or paragraphs. It was unusable for those titles. Not sure if it was bad OCR on the source file or what. I haven’t tried it on titles purchased from Apple, so I can’t speak to that.