it is completely viable to go all AGI with a fast weapon and destroy mobs quickly, the logic is sound more hits (fast weapon) more crits, more flurry, more WFs = fast killing… unfortunately at lvl 40ish the best gear available is slow weapons and str gear.
Let’s say your current critical rate is 10% and your current weapon dps is 100 (all factors being added in). You get a piece which can be either 20 Agility or 20 Strength.
That 20 Agility would give you +1% critical and +0.72% haste (from Flurry). This is approximately 1.72 dps with the existing dps.
20 Strength would be +2.86 dps (from 2 attack power per strength, 14 attack power per dps).
However, if the dps of your weapon (all factors considered) was 165 or above, then Agility is now equal to Strength.
That being said, let’s say you’re wielding Nightfall (a classic Enh Shaman endgame weapon). It’s 67 dps. To reach that 165 dps you’d need a whopping 686 Strength - about +600 Strength from gear/buffs.
In practice, Shaman will prioritize Strength > Agility almost universally (critical strike rating is better than both and should be sought out almost without regard to the other elements of the gear).
In terms of fast vs. slow, Stormstrike favors slow weapons - it’s one of the few attacks that didn’t get normalized in 1.8. One way to look at this is that Stormstrike provides additional ‘haste’ equal to ((20 / weapon speed) + 1) / (20 / weapon speed). So on a 3.5 speed weapon, it would grant the equivalent of 17.5% more attacks while a 2.0 speed weapon would only grant 10% more attacks.
There are two caveats here:
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In a raid setting, you don’t have the debuff slots for Stormstrike so there’s no advantage for the slower weapon.
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Slower weapons have more variance. In practice, this means you need to leave more ‘room’ below the threat being generated by the tank with slower weapons. This ‘threat cap’ is actually one of the most important elements of dps in raiding and it’s one of the reasons that hybrids don’t compete very well against Rogues/Mages - they don’t have the threat reduction talents those classes do. When your dps varies dramatically over time, the risk of surges of dps means you need to voluntarily curtail your dps.
It’s also important to note that in some idealized world, all you’re ever going to be is a glass cannon who never gets hit and deals damage. This is not how the game actually plays. Outside of raids, having sufficient armor to take a few hits is actually quite important - although BiS lists normally considered it to be of no value at all.