/NIN wins when you are pulling or when having easy access to shadows/sneak/invisi is important (ex nyzul)
/RNG wins when you want to do damage esp post 60. Little else beats 22 RACC/ACC, sharpshot, barrage, gun belt for a COR.
Each has it's role.
Though I would add that /nin is only need for pulling FAST mobs, if you cannot pull Normal speed mobs w/o getting hit, time to go practice pulling solo near a zone line.
Also /rng get beaters earring to go with the gun belt.
As to the mantle it sucks. I have one, and since about a month after COR was first Released I have been trying to sell it. Here it is years later, and no one ever bought it off AH or Bazaars.
To OP: What made you think /nin wins 30-60? /rngs acc bonus > 1 more racc weapon (and acc bonus applys to melee attacks as well, nice to get he tp).
On top of that, /rng get +8 Racc, +5 Ratt is SJ activated latent items at 30, /nin just get an AGL +4 earring (and its hardly better than a drone a few levels higher).
99 THF, COR, DRK, BLM, WHM, DNC