The most basic first; what presence should I be in and why?
Blood Presence is always the superior option. In the previous patch Unholy Presence was moderately competitive at certain gear points due to the GCD reduction aspect and the fact that our rotation was GCD constrained. Considering the latter is no longer the case, thus making the former now of absolutely zero value, the comparison between the two presences simply comes down to the damage verse the haste. 15% damage is an increase of approximately 12.75% dps, when you consider that it affects all of our damage except ghoul, gargoyle, and army. 15% haste, on the other hand, is only an increase of maybe 7.5% dps, as it only affects auto-attack (and auto-attack related, such as Necrosis, BCB, and Bryntroll) damage and pets.
Not just that, but Scourge Strike double dips into the 15% damage buff of Blood Presence, increasing its value and cementing it as the clear choice.
Double dips? I see that mentioned a lot in regards to Scourge Strike, but never an explanation of what it means.
The short, simple answer: If a buff or talent affects the damage of Scourge Strike once, it single dips. If it affects the damage of Scourge Strike twice, thus boosting the damage by more than the stated amount, then it double dips and is of increased worth.
The long answer: The new Scourge Strike is split into two actual hits; the first being physical damage and the second being shadow damage. The damage of the shadow portion is entirely dependent on the damage of the physical. What this means is that anything that increase the damage of the physical portion – be it a stat, a buff, a debuff, or a talent – will also increase the shadow portion by the same proportion. Outbreak, for example, increases the damage of the physical strike by 20% and by doing so, the shadow strike is increased by 20%, causing the Scourge Strike to do 20% more damage overall than it otherwise would, as the talent states.
However, some buffs and talents will affect the physical strike (thus increasing the damage of the shadow strike) but then also affect the shadow strike directly. Blood Presence, as previously mentioned, is one such buff. It will increase the damage of the physical strike by 15% which will then increase the damage of the shadow strike by 15%. On top of that, however, it will then go and buff the damage of the shadow strike by 15% yet again, meaning while the physical portion had its damage increased by 15%, the shadow portion had its damage increased by 32.25%. Thus the amount Blood Presence increased your overall Scourge Strike damage will be greater than the stated 15%.
Bone Shield, Desolation, Two-Handed Weapon Specialization, Blood Presence, Ferocious Inspiration, Sanctified Retribution, and Arcane Empowerment are all buffs and/or talents which are double dipped.