The iBook will last longer. Just because the system doesn't use as much battery power as a PB does. It's just a fact that iBooks last longer on 1 battery charge then PB most of the time, if not all the time. There may be exceptions.
Note that using something like MacTheRipper (Google for it) to transfer the movie - or, in particular, the feature only - to disk while the PB/iBook is plugged in will allow you to watch the movie off your disk drive and prolongs battery life a bit over using the DVD (mainly because the HD is often spinning anyway).
You'll probably get ~3 hours of movie time off a PB, up to maybe 5 hours on a 12" iBook.