iBooks are cheaper and lighter and have better battery life. Powerbooks are sexier. Powerbooks are "faster" (they have faster processors) and have faster componants.
For movie editing, you're going to want a larger screen, the iBook only goes to 14". You can hook up an external monitor to both, but the iBook is supposed to only support mirroring so we're talking 1024x768, however there's a spanning hack that might allow higer resolution.
If you must get a laptop for movie editing, get the powerbook. But those only come with 100gb harddrive and 1.67mhz. My friend goes to school for movie editing, and he uses the dual G4 tower with close to a terabyte of harddrive space. If you're serious about movies, you'll need a lot of space because uncompressed video takes a lot.