It depends on the processor. Those are so much lower because they are most likely Pentium M processors. An easy way to think of it is to double the ghz on a Penitum M. It'll be like a Pentium 4 roughly.
ex. 1.5 ghz M is like a 3 ghz Penitum 4.
It may not be exact, but that's a rough way to think about it...
The best, I'd say is Pentium M. Better battery life, which I think is very important on a laptop. However, it really depends on what you are going to use it for.