If you enable ATI PowerPlay then you would still get very good battery life. I get ~4.5hrs on my laptop with Max Battery profile (CPU clock on lowest multiplier - 800Mhz for Pentium M 7x0, 1000Mhz for Core Duo), PowerPlay set to max battery life and lowest screen brightness (can increase brightness and battery will last 3.5hrs-4hrs).
Gaming I get around 2hrs, full power.
BTW, 7400 has SM 3.0 whilst x700 only has SM 2.0 if that matters. If not gaming, go with the 7400.