They both have pretty similar battery life, apart from the 17-inch PowerBook beast.
An iBook can take all the consumer software, so iLife, iWork etc perfectly well (just make sure you have at least 512MB of RAM). iBooks are great for day-to-day use or as a second computer for one person. However, if you only want one computer or you want to use some of the pro applications, like Final Cut, Motion and others, then you're going to need the PowerBook.