I don't believe there is any difference in battery life between a Pentium M and a Celeron M based notebook. At least there hasn't been in my experience. Same with weight, not much difference between P-M and C-M.
The Centrino-style Pentium notebooks are lighter and have better battery usage than a regular Pentium/Celeron mobile. But unless you travel A LOT the added cost is not worth it. JMO.