roughly ... a P4 2.0 ghz = P M 1.4 ghz in terms of performance (Someone correct me if i am wrong)
But the advantage of a Pentium M is obviously the low power usage, which translates to thinner, cooler, lighter notebooks, with better battery life.
Its pretty much about ur priorities... If performance is of utmost importance to you.. get a p4...
If battery life and portability matter more... get a Pentium M.