Depends on the leader. If the donkey is a strong minded one and a clever tactician than I would definitely opt for an army of lions led by a donkey, but if the donkey is just...a donkey than yeah I would put my money on an army of donkeys led by a clever lion. He should know how to organize them to win the battle.

Bottom line is the leader that too often wins a war. Soldiers are just puppets played by the leader. If he knows how to play the game well then it's definitely a won battle.