Shaman Talents Released!

[Artega]

Crit for 1000 = normal hit for 500

500 damage you would have done anyway... therefore talent adds ~500 damage... he was being generous and saying crit for 1200 = adding 600 damage.

NS allows instant shock + instant chain lightning that gives chances to crit for much higher damage.  Or instant healing for added versatility. 

My shaman is only a baby at level 40, but I think I'd choose NS, primarily for the versatility of being able to be used for burst damage or for healing as needed.  Instant heals while following a flag carrier is a very nice thing.
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Of course, we're both ignoring the fact that you get more than just Elemental Mastery or Nature's Swiftness by going that far into that tree.

With EM, you'll be getting 1.5s Chain Lightnings (fast enough to be considered instant for most purposes as far as I'm concerned), 2.0s Lightning Bolts, and other goodies (like Improved Fire Nova.)

With NS, you'll be getting a nearly unstaggerable LHW, 5% boost to healing and lightning crit chance (you get a 6% with Elemental, which is why 30/21 can be so nice), and lowered totem and healing spell costs.

Personally I prefer my 31/20 Elemental/Enhancement build; I'm often able to kill my targets quickly enough that I don't find a need to heal myself or others, though I'm still fully capable of healing if need be. True, I can't drop a Healing Wave at the drop of a hat, but if we're talking about organized PvP (I've been talking about PUG vs. PUG since that's what 80% of BGs are), you'd be marked for silencing or destruction before you could fire that NS+HW, anyway. Assuming you've been healing frequently, at any rate.