The main difference between the two are graphics and, as you move up the range, screen size and obviously processing power. Also the powerbooks are a tad thinner/lighter and have some more supplemetary features.
Apple powerbooks generally have a faster CPU and a faster system bus. Also the graphics card is faster and has more memory. It is also easier to upgrade the Harddrive. Powerbooks are more geared towards 'professionals', whilst iBooks are for the 'consumer' market. Obviously your mileage may vary.