Moonkin 2t10/2t9 vs 4t9

I have been debating recently on whether to switch to 2t10 earlier or later - everyone should be doing it once they can do 4t10, but 4t9 vs 2t9/2t10 is a pretty open question.

This is mostly just to help me organize my thoughts on the matter.
4t9
Increases the damage done by your Starfire and Wrath spells by 4%.
The major advantage of this bonus is that it is reliable - it has no RNG component at all.  It also doesn't have any major disadvantages, but it is important to note that the talent is additive and not multiplicative.  For example, during Solar Eclipse 4t9 is actually only a 2.66% increase to Wrath damage.  This makes it relatively weaker then you might expect at first glance.

Math generally shows 4t9 to be worth slightly less than 3% dps.

2t10
When you gain Clearcasting from your Omen of Clarity talent, you deal 15% additional Nature and Arcane damage for 6 sec.
This bonus has a couple of interesting factors to it:

• If it works like other similar buffs, then the benefit will last the entire duration of a DoT.  A moonfire, IS or Starfall that you cast during a 2t10 proc should tick for 15% more the entire duration.
• You can gain Omen of Clarity automatically by casting Gift of the Wild.  This means that if you need to, you can use 2t10 to gain on-demand burst dps.  Since balance druids generally lack this, it might be valuable to explore (for example, imagine if you were having trouble on Marrowgar hard mode killing bone spikes - it would be very possible to hit gotw just before the cast finishes and get that little bit of extra burst for ~5 seconds).  Keep in mind that for overall sustained dps, casting gotw is going to be a loss unless you can do it while you're moving.  This isn't something you add to your rotation every six seconds - but that doesn't mean you should ignore it either, and hitting it right before the fight starts (as another form of pre-potting) may also be valuable.
• AOE - Omen of Clarity procs a lot when you're chaining Hurricanes.  2t10 should be a significant boost in damage on any fight with a significant AOE component.
  

 2t10 does have a few significant disadvantages:

• RNG, RNG, RNG.  Omen of Clarity is a relatively low proc chance (6%), and on average you will see around 2.5 procs per minute on a single-target fight.  But there can be a lot of variance in that number - the odds of going an entire minute with zero procs is roughly 8%, which is high enough to occur relatively often.  Other fights might yield you 4-5 procs per minute, it's all up to the dice gods.
• Six-second duration - spell damage is calculated when the spell cast finishes, not when it starts.  To get the 2t10 bonus you have to finish the spell cast within the buff duration, which means that calculating uptime on a six second duration is inaccurate - you will generally only get 5 to 5.5 seconds of useful buff time out of each proc.
  

 Overall, 2t10 looks to be worth slightly more than 3% dps, and significantly more on AOE fights.

Other Factors

• Set itemization - separate from the set bonuses themselves.  2t10 allows me to drop two items with spirit.  Even with the marginalization of haste past the soft cap, it is still very much superior to spirit.
• Item lvl - I still have mostly 245 t9.  Going to t10 gets me 251/264 items, which is a very nice stat gain.
  
• Experience.  Using 2t10 optimally means paying attention to the procs.  I have 1.5 seconds left on a 2t10 proc during Lunar Eclipse - do I cast a starfire (which will not benefit) or do I put up Insect Swarm, which will?  Since I will definitely have this set bonus at some point, the earlier I start watching it and incorporating it into my decision tree the better off I will be later in hard modes, when paying attention to dps will take a second seat to fight execution.

The experience factor is what decided it for me - if I can pick up 2t10 next week and start getting used to it now, then that's all the better for eventual hard modes..  Waiting for 4t10 will push that learning curve back another three weeks at least, picking it up now means I can start playing with tracking, addons and GotW right away.