Unfortunately, there are not a whole lot of head to head comparisons between the AXP-M and P-M. I can't do much better that restate what you probably know already:
*) AXP (3000+) will be quite a bit faster overall. A 1.4 GHz P-M would be in the ~2-2.2 GHz P4A performance range, the AXP would be more comparable to a (hypothetical) ~2.8-3 GHz P4A.
*) PM will have a noticably better battery life for equal battery capacity, especially when using low intensive stuff (light office apps, internet, ..) since AXP-M has a considerably higher standby power than PM. Using CPU/Harddisk/optical intensive apps (dvd burning, encoding, gaming, compiling,..), the difference would be less pronounced, but still very much a fact. Of course, battery life can always be traded with weight (higher capacity batteries), but if you want both (light and long battery), PM would give you a sizeable advantage.
*) you're more likely to find a nicely designed PM notebook than a AXP notebook. Choice is pretty limited with the AXPs, and especially in the "thin & light", there is a wealth of nice PM notebooks from just about any oem, and hardly any nice AXP based one's.
One thing though: if you decide for a PM based one, avoid the bundled "centrino" WLAN if you can. Its outdated, slow, and lacking essential security features like WPA. You're much better off with a third party 802.11g device, either built-in (if possible) or through PC Card. Its likely not even more expensive either.
= The views stated herein are my personal views, and not necessarily the views of my wife. =