First off, I said this before, and I'll say it again. If a Paladin knows what they're doing, and with proper support, there is no downtime. Now with a ninja, this is less dependant on his party. It's called mp management. If your using your skills right, you go heavy with abilities and spells at the begining, and then taper off at the end and allow your mp to tick back up, so that you don't need to rest.
Now, as for who is the better tank, it depends on the mob. An example would be last night, I got an invite to a party with RDM, WHM, BLM, WAR, adn NIN. I was kinda surprised, but I wasn't far from lvling, so I didn't care. And we had a tank-off against steelshells, and I came out on top. I ended up tanking the rest of the night, and we had no downtime. Hell, it was alot of fun, and we even stumbled onto a darkness sc(Blade:Chi > Rampage > Swift blade). Now, Ninja is an easier tank in most situations, but not allways.
And to those hammering on Isotope, chill out. The reason Iso is flipping out is that the mentality that ninja's are better in every circumstance is that same that made Beastmaster a solo job or that doomed many dragoons to lfp hell.