Assaults are usually very decisive, one side or the other quickly gains the upper hand and forces back their foe. Numbers and leadership can keep a side in the fight but the casualties each side inflicts is the most telling factor.
To decide who has won the combat, total up the number of wounds inflicted by each side. The side that causes the most is the winner, the other side is the loser and may be forced back if they fail a Leadership check. Note that wounds which have been negated by armour saves do not count, nor do wounds in excess of a model's Wounds characteristic, only wounds actually inflicted.