The performance of the M chips compares with P4 chips rated about 1GHz higher. The M is definitely the way to go. The battery life tends to be about twice as long as the P4 machines. It runs cooler which means less need for a fan, easier to design into a smaller case and doesn't roast your thighs

And don't discount the value of the Centrino wireless chiipset. It too is designed to be power thrifty. Laptop magazine does their battery runtime tests with and without the wireless connection on. The Centrino machines only lose a small amount of time when using wireless. Non Centrino machines vary wildly with some of them losing nearly half of their runtime due to wireless.

-Dylan