My 2 cents: Dark in a heartbeat. Here's why:
1. Dark's soul drain is MUCH better than Soul's soul drain. Dark recharges in half the time, can hit more targets, has a wider radius, etc. Given Scourge, this is reason enough to go dark. More damage = good.
2. Power synergies... dark pit + Opp gloom = stun fun. Dark consumption to help with endurance since you have no other endurance recovery powers. Soul transfer just cause.
There are only two reasons imo to go Soul over Dark: Power boost and soul storm. Soul storm could stack with Pet gaze for cool-looking effective single target holds. Although dark debuffs so well that I doubt having a single target hold will make that much of difference. Power boost really helps dark's debuffing however as it affects all of the tohit debuffing that dark miasma does.
So the question becomes, do you want to do more damage, stun, and resurrect at will? Go Dark. Do you want to quickly hold bosses and beef up your tohit debuffing? Go soul.
Personally, I'd go dark. Soul drain and scourge play nicely.
My dark/dark cor is now 42. I have completed my vig morality mish and am just about to start working on my villain tips with the goal of getting my patron powers. Before I begin, I ask my brethren to share their thoughts on which epic pool power will serve me best. I note initially dark offers an endurance recovery whereas soul does not. Soul seems to offer 2 build ups and a hold. Any thoughts, comments are appreciated. Thanks.