I use Bondo on almost all my mounts. Most of the time on larger game (elk, Moose, Etc) I will use earliners. It is easier to give the ears a natural shape with ear liners. I prefer bondo, because I have been using it for over 5 years and I have gotten good with it. Plus it is quicker than removing the cartilage and waiting for the ear adhesive to set up.
The trick to using Bondo is to add a good bit of fine chopped fiber glass in with it and NOT putting to much in the ear. Once the cartilage drys it gives it just that much more of a back bone.