both are used in auto body repair... both also work good on cabs..
kitty hair aka fiberglass is good for filling LARGE cracks and or fixing broken off corners.. its stronger but does not sand smooth..
SO .. if i was ficing big stuff id use kitty hair and then use bondo(aka auto body filler) on top to make it smooth..
regular knicks and gouges regular bondo works well..
(note, go to a local parts store that sells car paint and get some professional filler like evercoat or something simular.. it sands easier and finishes smoother then cheapie bondo does, and hardens faster too...)
advance auto parts even sells paint(most do) get thier "dynatron" filler its prolly cheaper then bondo and it works better too..