I have both XP and W7 on my LE1700. I haven't done real research, but I think there was no essential difference in battery time that I would notice. I use W7 most of the time, even when I have to spare battery time.
The different battery time of LE1600 and LE1700 is mainly due to the processor (and LCD screen) used. Core Duo-s take more power than old Pentium mobile-s. It also depends on which processor do you have, is is low voltage or ultra low voltage.