Disclaimer: My FC is in turn 5, I have tanked everything in the game on my Warrior and Twintania is the only thing we haven't killed with me tanking it (or at all yet). I also have a 50 PLD.
Bad Warriors will never be able to tank things that bad Paladins can. They will try, fail, and then blame it on the PLD having better cooldowns. Good Warriors have never had an issue tanking the vast majority of content (Dreadnaughts are about it). At the end of the day, a Paladin will run out of major cooldowns (here I include Hallowed, Sentinel and Rampart) eventually and at that point it becomes effectively a Warrior that can't heal itself and has lower damage/enmity.
Mathematically, PLDs are better spike tanks with the Warrior being the better consistant tank. But the gap is no where near as wide as most people think. Both tanks need to know when the big hits are coming, and both tanks will need to have something available to deal with those hits. For a PLD, they use a mitigation cooldown before the hit lands. For the Warrior they heal themselves either before the hit, after the hit or both.
Let's take Titan for example (because most people have done it). Mountain Buster hits every 30 seconds. A Paladin will be rotating cooldowns for these hits, and will have a cooldown available for most of them. But the third (or 5th if you use Hallowed for the 3rd) will hit and the best cooldown you have is Convalescence. For that hit, the Paladin is a much weaker tank. This pattern continues, occasionally leaving the PLD 2 MBs in a row with nothing to use. Meanwhile the Warrior has Inner Beast available for every single attack, and twice on every other. Berserk, Featherfoot and Second Wind can be used for every third MB, with Internal Release being used (every second MB) on each of the double IBs. For the Titan fight at least, once you have a minimum amount of gear (lets say full Darklight) the Warrior is probably the better choice for that fight, as it handles the Mountain Buster damage spikes much better and everything else is trivial.
I expected people would have figured this out by now, but most people simply look at average mitigation over the whole fight, and if that is all you consider, then the Paladin wins hands down. In practice, there are periods where a Paladin simply has no cooldowns of note, and the Warrior becomes the better tank for that period. That amount of time can be up to a minute in length, depending on how good/bad your PLD is at rotating his buttons.