conserving battery life

[Video International]

Here's one that may stump ya.

I'm taking a long flight and want to watch some personal inflight 
movies (tv shows actually).
Which will give me more viewing time running on a battery?

a) watching mpegs directly from a data DVD.
b) copying the mpegs from the DVD to my hard drive to play
(will have to delete afterwards due to limited space).

Anyone know?

Rick