Mobile chips save power, especially the Pentium M processors. A notebook equipped with Pentium M will run for 4 to 5 hours, while a Pentium 4 M will run for a little over 2 hours. Notebooks with a desktop Pentium CPU will run only 1.5 to 2 hours. Speed wise, a 1.6G Pentium M is almost as fast a a 2.6G Pentium 4M.
Which one is better depends on how you use your notebook. If you take it out often, then you'll need a Pentium M, or at least a P4M notebook to have longer running time. But if you use it as a desktop replacement and leave it home most of the time, then getting a notebook with a P4 3.2 G CPU will give you the fastest speed.