Zulu longswords are tougher than Impi, and they upgrade to 3-move muskets, while the Impi have to wait for Rifling. If I have the iron for it, I always build a few longswordsmen for drawing fire from cities while my trebuchets set up and the Impi run around killing everybody. I seldom build Zulu regular swords other than upgrading my starting warrior, who doesn't have the buffalo promotions so he pretty much stays home for defense.