Hi Burr
First I have learning that Zbrush use also "Voxels" when it make an "unified skin" above a volume!
But user don't see it and don't know it ! :)

Difference between the 2 progs is now, a more fludidity use of fonctions during modeling for Zbrush, more natural and speedy!
A more geometric possibility functions use for 3Dcoat ! You have curves!
They are measures inside 3Dcoat! And more accurate for hard Sculpting and Box modeling!
Better tools of Retopology

Learning curve is more speedy for 3D coat (as more traditional UI)

If you are artist Zbrush, if you are engineer 3D Coat :)