Default clockspeeds:
2600+ Barton => 333 FSB, 11.5x166 = 1.9Ghz
2400M Barton => 266 FSB, 13.5x133= 1.8Ghz
If you don't plan on OC'ing, then the 2600+ is faster slightly in clockspeed, but the higher FSB makes a big difference. If you plan on OC'ing, then the 2400+ Mobile blows the desktop chip out of the water, because it could probably hit 2.2Ghz with an even higher FSB.