I think its mostly legacy. xnormal precedes marmoset. and its free. marmoset is worth the cost IMO for the baking alone. It just makes it very fast and easy. I like the new texturing workflow in marmoset a lot as well. I prefer it to substance, though admittedly it is not as mature. But for my needs it suits me fine.
I was just doing a tutorial on creating hair for real time characters and the tutorial used XNormal to bake ID, normal, AO, and height maps. It took an insanely long time to render them all, around 4 hours total, with the AO map taking 3 hours of the total render time. I suspect that maybe my poly count was too high…
there is at least one person here who is a regular name and seems to be specialized in hair. I cant remember now. Thomas something? Might look through the sketchbooks, you might find some tips from them. it may be that xnormal can bake certain types of maps that marmoset cant. Or just that guy already knows it fluently so…