They are different products designed to do different things...
WFG is actually a fine abrasive polish meant to clean up minor defects or defects caused by a more aggressive compounding step. (Primary use machine)
WGPPE is a pre-wax cleaner to clean off old wax off the paint and clean some bonded contaminants off the paint. (Primary use hand)...
Depending on what your doing - you won't need both.