Both have their ups and downs. So here's my 2 gil, for what it's worth.
RDM/NIN - Blaze Spikes, En- buff with dual weapons, Regen, Stoneskin, and Phalanx makes you a serious gil farmer who can decimate a LOT of easy con mobs.
Somewhat limited in groups, though, as you tend to use your mana more than your melee, especially at higher levels. By not subbing a mage subclass, you lose out on alot of abilities, as well as improved manapool.
+str and accuracy items are your friends.
NIN/RDM - so far I've found it works fairly well in groups. I can combine the RDM buffs with Ninjutsu to debuff the hell out of enemy, plus have the leftover mana to act as constant puller (Dia pulling is great), and help a bit with healing (not a lot, obviously, but a BIT :p). Downside, obviously, is no Provoke. At later levels many view a Ninja without Provoke (to blink tank) as useless, so that may hinder you later on.
Juices for mana regen are your friends.
End result - I'd say both are "novelty" combos. They're not the BEST combo for each class (generally RDM/BLM and NIN/WAR work best), but they can still work, as long as the group doesn't mind.
They're damn fun though
