I leveled thief and dancer to 90 after leveling my monk (my main job). I thought I would level thief to have TH for parties, and I wanted to be dancer for soloing.
Then I leveled ninja later on, and now when I solo, I go nin/dnc instead of dnc/nin. You asked which is better for grinding and for NMs. Well, for grinding normal mobs, any job can do the trick. If we're talking about abyssea, I prefer thf/nin for that because I'm usually farming pop items of some sort and I want the TH for increased drop rates. If you're talking about solo farming NM's in abyssea, I prefer nin/dnc over dnc/nin.
There are some interesting distinguishing factors between nin/dnc and dnc/nin:
-When a ninja subs dancer, he gains benefit to a pretty nice job trait - evasion bonus II. The evasion bonus job trait added to your high evasion skill is really nice and I see a noticeable difference in my evasion when I sub dancer instead of warrior. Dancer doesn't get any useful job trait by subbing ninja.
-When a ninja subs dancer, he not only gains the ability to heal himself with Curing Waltz 3 for ~350ish, he also gains the ability to stun, which is paramount for soloing NMs with nasty AOE spells. BTW, most dancers still use Curing Waltz 3 instead of Curing Waltz 5 because of the delay timer between the 2.
Overall, as with most jobs, when you sub ninja, all you gain is the shadows. For some jobs, like THF, you also gain Dual Wield 3 and Subtle Blow 3 which are both very nice for THF. One thing I don't like about soloing NMs on THF/NIN is that your only reliable way to heal is through either bloody bolts, temp items, and/or regen atma. The other main thing is that most THF job abilities are not for soloing, especially your 2 big damage job abilities: SA and TA.
One thing that I don't like about dancer, is that, even though they can swing fast with dual wielding and haste samba, a lot of their auto attack damage gets cut down by constant usage of job abilities. You're constantly using Steps and Flourishes to skillchain. It usually goes something like this -> Presto, Step, wait 10 seconds, Step, build TP to 100, WS, Flourish, WS. Rinse/Repeat. Not to mention you're casting Utsusemi to keep up shadows, rebuffing samba, and waltzing when you take damage. All that cuts in to a lot of auto attack damage.
In abyssea - ninja has been given a great opportunity to do extremely well. Atmas and cruor buffs allow players to crit on most of their hits. Ninjas are given access to more gear that increases crit hit damage than other jobs. (Example, my non crits will do somewhere around 75 damage, but my crits will hit for around 200). The empyrean +2 boots gives ninjas 5 shadows when using Utsusemi Ni and 4 shadows when using Utsusemi Ichi. If you think having 6 shadows as XXX/Nin is awesome, try 9 shadows as Nin/XXX. I'd say 90% of the time, I don't even use Utsusemi Ichi anymore on my ninja, which also allows me to TP a little bit faster by not having to use the longer casting spell.
On another note, why don't you just level all 3 jobs? With all the increased ways of getting xp, it really doesn't take long at all to level up a job. Several months ago I wanted to level my ninja just as a subjob for my level 80 monk. I got to level 30 and hopped in an xp party with the intention of leveling it to 40. It was a Saturday morning when I got in to an abyssea konschatt lizard party. Later on that afternoon I was level 72. A few months later, when I actually decided to try playing ninja as a main job, it only took me a few more hours to get it to 90. Now that they have Grounds of Valor and have doubled the xp rate for non-abyssea zones, leveling to 30 is even easier. And getting to 90 from there is simply getting in to an abyssea party and grinding it out for a day or two.
Personally, I play my ninja more than my other jobs, followed closely by my monk, both of which are fully +2 now. My dancer and thief are my other 2 level 90 jobs, both with full +1, but I rarely play them. I usually pull out my dancer to go work on quests/missions because I like running around with Chocobo Jig. On rare occasions I'll pull out my THF to farm pop items for my monk's empyrean trial weapon. I have a friend, who also had 4 jobs at level 90. A couple months ago he decided he wanted to get Maat's cap. It only took him 7 weeks to level every job to 90 and level up the skills to fight Maat. He said "it takes longer to level up combat skills and magical skills than it does to level from 1 to 90." And it's true.
Now, you're talking about nin, dnc, and thf. Dnc and Thf share the same weapon skill: dagger. So if you level one of them up, leveling the other one up won't require you to work on any extra weapon skills. If you level Ninja, you'll have to level up katana and ninjutsu, but dual wielding katanas and spamming ninjutsu spells will skill you up very fast, a LOT faster than skilling up the two-handed weapon skills that you did on your warrior.
P.S. Sorry for the extremely lengthy post. I'm killing time while I wait for my next class lol.