I've played both and honestly, Machtaru has the right idea on this one. NIN and PLD pts have to play completely different to unleash their full potential.
The only advantage of being a NIN is I don't have to sit between mobs, and the puller can pull whenever they choose to. All DD's have to stay below the hate threshold as a NIN, which I honestly dont like doing(even with a THF, a good WAR can pull SATA hate off me with provoke and warcry). I do better damage as a NIN, but my parser only shows about a 5% difference between the two job classes so i consider my damage with either class to be a bonus and nothing more.
With a PLD tank, yes I have to rest between mobs, but isn't that why SE gives us a small amount of time between chains? I need a smart puller rather than smart DD's when I play PLD...someone who knows when we need to kill the next mob to keep the chain going, but lets me get as much MP as possible between fights (not going too fast so I run out of MP). I love playing PLD because hate is never an issue, and DD's (including BLM :D) can go all out!
As always, it's not that one job is better than the other, it's all about who is behind the screen on the other side. Anyone who would not invite me because "PLD's suck" or "NIN can't hold hate" can go ahead and keep looking for their choice of tank. Their loss.
I honestly reccomend for all of you that do show a preference for either PLD or NIN to try both jobs and discover their strengths and weaknesses for yourself. It can be an eye-opening experience.