1.5 vs 2.4 ghz refer to the clock speed of the CPU, but CPU speed isn't everything.
The Pentium M model is specifically designed to be a low power consumption model, which means it probablly offers neat features like clock throttling, which should ultimately lead to longer battery life.
The 2.4ghz is a faster, more beefy machine, and is probablly worth the money in terms of raw CPU crunching power, but unless they're doing something else to help with the power consumption, you can probablly expect longer battery life out of the 1.5M. Of course the argument goes you can get more done in less time with the P4.
So, what's better? Depends on what you need. What's the primary purpose of this machine? Word processing? If so, cpu speed is your last concern. Gaming? I'de jump straight to the 2.4ghz and start being -really- concerned about the video card. Video/sound editing? Get a mac instead. You'll thank yourself a thousand times.
Heck, get a Mac anyway. They're dead sexy machines with an awesome OS (OSX).
-Kevlar