The main difference between status-area-applet-battery and Advanced Power is that the former is a built-in part of Maemo whilst the latter is an additional piece of software. In CSSU-Devel there is a replacement of the former which shows a bit more info, making the latter redundant. As a rule of thumb, I use Advanced Power until I "upgrade" status-area-applet-battery to the one from CSSU-Devel, when I happily remove Advanced Power to save space and memory.

Why is this discussed in the Thumb thread is beyond me though.