Originally Posted by
SystemTheory
The LiPO will have a more constant Rs until maybe 80-90% depth of discharge, so you get more of the stored energy out of the pack (extended run time).
I may be in error about that 80-90% figure, it may be constant to 100% of the discharge capacity.
This page shows a graphical comparison of NiMH to LiPO from a few years ago, where the technology is improved but the method of comparison is sound.
http://www.slkelectronics.com/ecalc/gen4a.htm
According to this source a little warmth in the LiPO actually reduces internal resistance Rs, and it stays relatively constant up until the cut-off voltage. Thus LiPO has a dual advantage of higher energy density and lower internal resistance per nominal volt.