It doesn't matter which one you use to set it. ASUI does use a lot less memory than adv-power + systemui and I hope it is more efficient but not going to make any claims on that.

Powersave might actually drain the battery faster when running tasks that put the processor under full load since it spends less time idling. It might help if you don't do anything that would normally keep the processor at 400mhz for more than a second. -- Changing from full brightness to very low brightness cuts the battery drain in half. Setting op_dsp to 0 can increase battery life if you play music (other than OGGs) or watch movies (only with media player). Other than that, not running apps that continuously run (games, movies, metalayer-crawler) is the only way to really increase battery life.

ASUI has a battery drain meter that should show 0.4 %/hr (minimum) after your device has been idling for an hour or more with screen off. And if the marker above the ASUI battery applet remains gray when reading and only jumps to yellow when scrolling then you're getting maximum battery life.