Dan,

Whether you use NiMH cells or Lithium cells is a design choice you'll have to make when you determine the space available in the fuselage. Since you may not find a Lithium cell that can fit inside the fuselage, you choice would then be NiMH over Lithium or NiCd. Today, there are so many new NiMH cells available that you can rule out using NiCd cells.

Here is the typical energy density of each cell type: