In this mode, the Portable and PB100 will get 64 wait states for every memory access, bringing the effective speed down to something equivalen to a 1Mhz 68000. The result is about the same. The PowerBook 170 can run at 25Mhz or at 16Mhz. If you are not really interested in a fast computer, you can save some power by running at the lower speed. Since I don't have a PB170 of my own, I can comment on which way I would run it.
Battery life is such a hard thing to bargain with.
My PowerBook battery life is nowhere as long as advertised. If you only use the battery halfway and then charge it, you run the risk of producing a memory effect on the battery. Apple has tried to eliminate the problem, but you still should try to use batteries until they are empty. On the PB100 and Portable, you should never let the battery discharge completely. If you do, the capacity of the battery is reduced and the damage is usually irreversable.