Powerdown Mode in most systems will save the most power. Most derivatives require a reset pulse to recover from the power down mode. For these systems you need to devise a way to get that reset pulse. It could come from a user ON button or from some type of sensor that has been triggered. Some derivatives these days also have a multiple number of ways to derive a RESET signal right on board the microcontroller. For example I have read about a part that has an available voltage comparator on several user pins that can be setup to cause a reset.

There are also a number of deriviative type parts that now allow recovery from power-down mode via an interrupt condition similar to the way that the IDLE mode recovery works.

Note that I actually ran into some trouble with one project I took on to make modifications to an existing product. The device used POWER DOWN mode as its OFF state and depended upon an interrupt from a user press of a switch to come back ON. The first alternate source parts I purchased to put my new code into were 89C51 style parts without power down recovery from an interrupt (much to my chagrin) while the older 87C51 type parts had supported this mode.

Michael Karas