for filling directly on wood, this is one of the best I have used, Minwax StainableWood filler, when I use it for modeling, (I use it for work as well) I thin it ever so slightlywith a little water to loosen it up and make it a little more spreadable.

a photo of my latest project,my FockeWolf 190A8,the tail and wing fillets are filled with the Minwax filler.

after its dry the filler sands well and is very tough,its also light weight like the spackle but much more durable, and has a wood tone so it blends with the balsa color well.

as for Bondo type fillers I would have to agree with the posters that its needlessly heavy, and tougher than the surrounding wood areas wich can be troublesome when sanding it back.

a exellentpolyester filler for filling fiberglass, and glassed surfaces, like a fiberglass covered airplane is Evercoat body filler http://www.evercoat.com/productDetail.aspx?pID=33another material I use alot of for modeling and work, itsands smoother than regular bondo and is lighter for modeling.

I generally use it for doing the fine detail work after glassing my models,so there is minimum weight added.