Heart of the Wild is out because a) I'd only get to use it once and b) healing done while not buffed by Tsulong's breath is considerably less useful. I could probably get off 2 HTs per breath instead of 1 using Dream of Cenarius and HotW heals are twice as strong as DoC heals, but HotW only lasts for ~1/3 of a single day phase, so I'm pretty sure DoC would still be competitive in terms of healing done to Tsulong (though a HotW and breath buffed Tranq would probably be pretty darn cool).
Currently I'm leaning towards Narture's Vigil because I think it'll help out people who struggle during night phases a little and I will be able to devote all my effort to killing adds during day phases (LFR groups seem to have particular trouble with adds in my experience, but the point also stands for standard raids). Whereas Dream of Cenarius wouldn't provide much benefit beyond a slightly speedier fight. Since mana shouldn't be an issue (the breath also restores 25% of max mana), you really only have to worry about the enrage timer. The final push before the enrage timer hits is healing based, though, so it might behoove me to assist in that.
Any opinions would be greatly appreciated. Both are good options, but I need someone to tell me to go with one or the other so I can stop agonizing over it.