I am putting my money on the Centaurs, given that their blessing is a lot more effective, and they should deal a lot more damage than the Vans (no Flaming Weapon hurts them). So... 20 centaurs killed, to pull a number out of my hat.
I forgot that the centaurs already start with +3 Berserker Rage: So i think they will win with 5 dead centaurs after the fight(protection 23, if i am right) and that the white centaurs win.
The centaurs will absolutely dominate the vans.
They won´t tire and a van hoof attack will only do about 5 damage to a centaur, the spear attack has no chance of going through the armor.The centaur almost regenerates that amount.