Eh, again, MNK being generally ahead of NIN for tanking is just an assumption.

Code:
Job	D Main	D Sub	D Kicks	ν Main	ν Sub	ν Kicks		ν DMG		TP/hit	ν TP	ν WS		WD Main	WD Sub	fTP Main	fTP Sub	WS DMG		ν WS DMG
Monk	93	93	104	0.47732	0.47732	0.068536	95.90926	5.3	5.42283	0.0542283	217	217	2.73809		3.35559	1322.32856	71.70766688
Ninja	70	51	0	0.79415	0.79415	0.000000	96.09215	4.5	7.14735	0.0714735	213	194	4.40059		1.45559	1219.71013	87.17695198
I don't have the energy to come up with a long winded explanation of everything here, but you can probably figure it out by looking at it and thinking for a while (ν means frequency.) It's my Kannagi NIN/WAR vs my Verethragna MNK/WAR in a Heroes zone with Haste only.

Minor things were ignored (such as AF3+2 set bonuses and WS TP), but it does show that NIN & MNK melee DoT are nearly identical. MNK has the upper hand for average WS damage, but NIN has such an advantage in TP gain that it makes up for and overcomes this. So, in the case of disregarding casting times and ability stasis, then NIN is the winner in sheer DoT potential.

These comparisons come before pDIF is calculated, meaning that cRatio, critical hit rate and critical damage bonus are not considered. However, the difference in attack between these two jobs is actually quite small and the critical hit rates are going to be identical unless the MNK is under Impetus. NIN can use Innin in many situation though, which will essentially ceiling your critical hit rate at 95% (only on mobs that you can bank tank.) MNK has a base of +30% critical damage bonus (which can be improved during Impetus), NIN has +45% for melee and +50% for WS. Either way, it's difficult to say which job has the advantage in regards to pDIF because there are so many unpredictable variables, but I if I went from my gut feeling I'd say that NIN has the upper hand usually.

This also ignores yet another variable, perhaps the variable, which is Counter. If MNK DoT potential ever exceeds NIN DoT potential (with just Haste spell, that is) then it's because of this. It's difficult to really quantify how much it helps, but consider that most mobs have a delay of 240, or 4 seconds. Being generous and giving MNK a 70% base counter rate (50% Counterstance, 10% Melee Gaiters, 5% merits, 5% Atma of the Gnarled Horn) yields a ν Counter of 0.16625, which essentially adds 15.46125 to ν DMG. This isn't a very reliable value though because it ignores mob double attack/triple attack rates and assumes capped player accuracy (which is common but not certain) and also that your evasion is floored (this almost never occurs.) It's very much subject to change, but it's interesting to note that this 'reference' value, when added together with ν DMG & v WS DMG, is still not enough for MNK sheer DoT to overcome NIN sheer DoT.

Either way, I play MNK & NIN extensively in Abyssea and have both Empyreon weapons. Is this what I observe in terms of sheer DoT, that NIN is greater than MNK? I wouldn't say so mostly because of how cast times affect it (although this has been said already.) I just wanted to show that, if a NIN disregards casting spells and relies on cure bombing much in the same way that MNK does, its sheer DoT potential is equivalent to or often greater than a MNK's. So if that's your criteria for which tank is better (i.e. killing the quickest while being able to survive), then NIN is at least on par, certainly not behind.