I'd say that as a ranged spec with a lot of instant abilities (like Moonfire), doing the repeatable quest is quicker. If you're a melee, you're probably better off to farm the instance completely (there is Hibiscus beyond the Naga section). Mobs can also drop the Hibiscus, and if you loot an entire instance, you won't finish it in less than 12 minutes.
If you want to grind Cenarion Circle without stopping until you hit exalted then grinding Twilight Texts is the way to do it. If you are not in a rush then I suggest just clearing AQ20 every week. Full clear gives you several thousand in rep. I started doing that on my rogue and after a few weeks hit exalted, I don't remember how long it would take me but I know it was less than an hour, all depends on how much trash you kill along the way. Pretty sure several bosses in AQ40 also give rep.