If theres one thing about Ninja and Paladin tanking its more about Risk and Reward. A WAR/NIN at 74 will outshine just about any Ninja in many aspects but cannot compete with holding hate with a PLD at least not without alot of enimity gear.

NIN, if its a good one, seems to chain exp faster and you can take stronger and more IT+++ kind of monsters. However they suck at holding hate compared to the PLD and you can pull hate more easily especially for RNGs and BLMs.

Now for PLDs you cannot take those really tough monsters that hit like powerhouses, but its more reliable, i can unload and not check my hate as much with a PLD tank and deal more damage than i would ever dare than with a WAR or NIN tank and that in itself can also get more exp with the only bad part that you spend more MP and are more MP dependant. But the thing is, you dont die :P