I don't remember exactly, but I remember people saying that while the G5 has the much higher bus speed (cpu::chipset, cpu::memory is from 167 to 400), the G4 would probably win out if the clockspeeds were close at all. If going to the PBook is for portability or everyday use, I'm sure the G4 1.5 would be about the same.
As a desktop replacement, the HD's in laptops in general will always be slightly slower. But aside from that, laptops are easily catching up in other categories (vid card or memory for example)