It depends on which version you're talking about. Well, I'd still prefer the Ninja, but in the PS1 version that preference would be greater. In the NES version, Black belts/Masters really only save money. Ninjas add a bit more flavor, and are a nice balance to a Knight's limited white magic abilities.

However, going on purely practical concerns, a second Fighter is superior to both. Much better defense and relatively similar attack to the Master until higher levels where it hardly matters anymore.