Combat build critique

[Gurnsey]

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Not being able to use poisons with swords is a pain, but I’m not interested in using daggers (which also rules out Backstab/Ambush).  I’m hoping the 5% from Sword Spec will proc quite often, can you crit on these extra attacks?  If so, a 5% change for an additional attack with your normal crit chance is far better than an extra 5% crit.

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Let's look into this question from the perspective of a Sword/Sword SS build.
Is 5% crit better, or 5% chance of an extra attack?

Example:

50 dps base.
15% chance to crit (not hard to do).

Base

The crit chance (over time) effectively adds a 15% chance to replace a normal hit with a 200% damage hit = .15*50 = +7.5 dps.

Total Base: 57.5 dps

With Sword Spec

This adds a 5% chance to do an instant-extra swing (does NOT replace a normal swing) with the normal chance to crit (I don't have direct experience with this, but I crit with my Shaman's Windfury all the time and it is very similar) = .05*57.5= +2.875 dps

Total with Sword Spec: 60.375dps

With +5% Crit Chance

This is just a modification of the calculation of the base DPS. (.15 + .05)*50 = +10 dps

Total with +5% Crit Chance: 60 dps

It looks like Sword Spec is the winner, but not by much.

Going Beyond

Though it's not a huge difference in numbers above, there are some special factors to look at with your build. An 'SS' (Sinister Strike) build favors Damage Per Swing (slower, harder hits) over Damage Per Second, and Sinister Strike favors heavily into the build's damage output. I'm almost completley sure that Sword-Spec will not proc on a Sinister Strike; SS can crit though.

This partially relates to 'Streakiness'. Since the build favors slower weapons, you are making fewer swings in a certain amount of time. A 5% chance (1 in 20), swinging 20 times in 60 seconds, should happen once. However, it is quite likely not to happen at all, or happen a twice. In our Sword Spec example, this has a likely possibility of not adding any dps; or, it might add significant extra dps! Streaky, especially since it doesn't add anything to Sinister Strike. In the +crit chance example, the 'chances' happen on both the normal swings and on the Sinister Strike swings. This gives you more swings or chances per time period, reducing the 'Streakiness'.

Also, since +crit chance applies to Sinister Strike, but Sword Spec does not, the TOTAL dps calculation becomes:

Sword Spec: 1.05*1.15*(base dps) + (SS dps) + .15*(SS critical damage bonus)*(SS dps)

Dagger Spec: 1.2*(base dps) + (SS dps) + .20*(SS critical damage bonus)*(SS dps)

The last term (the added dps from Sinister Strike crits) is lesser in the Sword Spec equation. Thus, the talents that increase the critical strike damage bonus of Sinister Strike add more value to the +crit chance scenario than to the Sword Spec scenario.

Since we are assuming from the first results that Sword Spec is more valuable than +5% crit chance, let's subtract the two equations. We then come up with:

{Sword Spec} .0075*(Base dps) = .05*(SS Crit Bonus)*(SS dps) {Dagger Spec}

Since you plan on having the talent that increases the SS crit bonus maxed out, we can reduce this to

{Sword Spec} .0075*(Base dps) = .05*1.4*(SS dps) {Dagger Spec}
{Sword Spec} .0075*(Base dps) = .07*(SS dps) {Dagger Spec}
{Sword Spec} .11*(Base dps) = SS dps {Dagger Spec}

So, for the +crit chance to give you more dps for an SS build, Sinister Strike damage-per-second needs to be at least ~1/10th of your base dps. Something tells me this is not particularly hard to do. Every bit of dps over 1/10th of your base further adds to the crit-chance advantage.

Whew. That was a lot of algebra! See reply to myself below for some complications.

[edit] Crit damage is 200%, not 150%. Thanks for catching me! This made crit-chance work out to have a better advantage than before!