P. marginalis has a fawn coloured reniform (kidney shaped) mark with a white bar adjacent, P. testaceicollis has mainly a white reniform mark with a tawny bar adjacent; P. marginalis has a large white orbicular mark and another white spot near the forewing base, P. testaceicollis has a smaller white orbicular mark and no additional spot at the forewng base; P. testaceicollis has a uniform brown "collar" - a band of scales across the front of the thorax, P. marginalis has a strongly banded, not uniform collar at the front of the thorax.
I thought they looked different but then you get something like the Elhammas and there is a whole bunch of them that look distinctively different but they are all the same species. Or someone hasn't worked out yet that they are not. I thought this moth (Proteuxoa testaceicollis) was particularly beautiful.
Yes, there is a large degree of variation in some species, whilst very little in others. And sonme families and genera are much better known by scientists than others. There are good notes in the NatureMapr field guide for P. testaceicollis, distingishing it from other similar proteuxoa (genus) species, but those notes don't cover P. marginalis. Notes about identiftying P. marginalis are on a supporting disc to Moths of Victoria (Part 9), available for those who buy thr hard copy volumes of Moths of Victoria, but they are not available online unfortunately. I agree, P. testaceicollis is a stunning looking species.