I'm no expert on processors but i believe the Pentium P4 is more optimized for battery life than processing power. Where as the Pentium M is more a compromise of performance vis battery life. I think this maybe the reason that if you go for the P4 you have to have the faster processor. IE Pentium 4M 1.7 mhz is out performed by the Pentium m 1.6.

I no this to be true of at least the Dell made notebooks as i have one.

As for what you should go for that is dependant on your budget. Dell are more expensive than some of there competitors and not others. If you can afford the more expensive laptop i would go for that, for the following reasons.
1/You will have a more powerfull system.
2/The system they are offering above has been around for some time now, and as Nihil has said it looks like a P3. As the P4M is new technoledgy it may have some upgrade options available at a later date. If it is the system i think it may be, you will i believe, only get the 64 meg graphics card, But it will take the 128 meg card.

This is just my thinking you realy have to make your own mind up.

Jinxy