Aedric Spear seems to be capable of slightly higher Single Target damage. However, the Burning Light passive that contributes to a lot of the damage does not trigger in AoE. In addition to the bonus weapon and spell damage, Aedric Spear also gives a bonus to armor with the Balanced Warrior passive that you would not have access to with Assassination.

Assassination has a ton of extra critical chance which is great for both Single Target and AoE damage. The passive from slotting Soul Harvest (Death Stroke morph) offers an insane amount of Ultimate generation in fights with lots of adds. This line also offers VERY GOOD sustain for fights with lots of adds due to the Executioner passive: When an enemy dies within 2 seconds of being damaged by you, you restore 1000 Magicka and Stamina. You also have access to the Hemorrhage passive which gives you and the group even more critical chance IF that is not already provided. This line does fall off a bit for fights where you can’t flank, as you lose 6.6% of your critical chance.

Both lines offer extra critical damage and both are very strong for DPS in general. I think the Assassination line is more versatile and the option I’d go with more often, but Aedric Spear is really great too.