In EXP Parties, Ninja tanks are much more effiecient, but require much more care than Paladins. Paladins really only need to be cured and get MP regen, they can still hold their own without being hasted and having status effects removed. Ninja really was never meant to tank, so the conditions they tank in must be near-perfect or there will be lots of blood and tears, but with the right conditions you can have yourself a very good party. I'm not even going to mention damage dealing because it's pretty obvious who the victor is. Unfortunately, in most endgame parties, Paladin is useless. The mobs only last a few seconds, which is enough for a Ninja to handle, and they can rack up an insane amount of damage in the process.
As far as HNMs go, it's up in the air really. Paladins can take a fuckton of hits, get curebombed, and still hold hate like a champ. They can tank virtually any HNM with a dedicated healer and refresher. The thing is that they will take every single hit thrown at them, so if one of those collosal moves is used at the wrong time, you're effed in the ayy. With Ninja, same as EXP Parties. If your mages are on the ball, and the person playing Ninja knows what he/she is doing, there isn't much to worry about.
Kiting HNM, both can do it very well, but Paladins are more reliable because of all the hate grabbing abilities. I knew a Ninja tank that could kite Kirin full time only losing hate to rangers and chi-blast monks, and would only die to the 2hour, or sometimes would never die.
Dynamis. Paladin main tank only. Ninja can kite stones, or DD, there's only one star in this show, though.
