First of all, if it says XP-M; then it is an Athlon XP-Mobile. Barton core, I think like 35W power dissipation, 512KB L2 Cache, 333MHz FSB, etc. The Pentium-M will perform better and have better batt life. However, the XP-M is no slouch on a notebook for general useage. With a 15" screen on lowest brightness using wireless you can expect ~3 hours on a 6-cell batt; not too shabby. And the XP-M based notebooks are dirt cheap now. The biggest manufacturers of them are HP/Compaq & eMachines.
If you can afford it, definitely go with the P-M tho. Better perf, better batt life, and overall an astounding piece of computer architecture.