Honestly if it actually happened I think be would be so one sided and simply Tauren Victory.
The corpse army would simply be crushed under to hooves of the Tauren, noting would be left but dust.
Now if the Forsaken had orcs or other races helping them different story.
And if we had no player involvement I just think the Tauren would crush them + I am sure the High montain Tauren would side with the Mulgore Tauren anyway what do you think?
Maybe I am just more a fan of the Tauren I just think they are the strongest race within the horde.