Unless you have high amounts of crit and mastery a single eye beam beats or atleast ties with 3 x epidemic. Epidemic damage caps at 315% AP per target, while eye beam does 550% and always crits, so effectively 1100%.
Fel rush does another 441% each with fel mastery.
With Unholy's other tools you can get ahead after that right now, since DnD AoE + DT timmy cleave + possibly bursting sores also does terrifiying amounts of damage.
But then again dh is far from complete and has a miuch stronger emphasis on AoE on tier artifact than unholy does. All 3 major traits of havoc benefit from AoE.
Fury of the Illidary is a really brutal AoE nuke, which gets boosted by 60% by rage of the illidary and while that is split damage it still counts the AoE you did effectively being AoE that way, too.
Inner demons is also AoE and while random you can say the same about shambler for unholy. Followed by Anguish of the Deceiver which is another 400% AP extra for eye beam.
Looking at the minor traits we have balanced blades which is 10% extra for each target in range. 15% more glaive damage and while only 3 target cleave the glaive hits pretty damn hard., especially with blood let. You have another 18% on eye beam which is huge for AoE.
Unholy only features portal to the underworld (very huge potential but hard to control) and plaguebearer directly and the extra 10% shadow damage and improved fallen crusader indirectly as it simply improves general output while havoc doesn't have such artifact traits.
Both can now brutally invest in AoE on their talent tree.
The initial few gcds havoc should definitely win, while unholy should close in and overtake havoc the longer the AoE lasts due to havoc AOE being linked entirely to CDs and the most significant AoE is on higher CDs for havoc than for unholy.
If havoc goes all out with demon reborn and throws you 6 x fel rush + double eye beam + fury of the illidary which you can only do with meta then havoc completely destroys everything on AoE but thats on ahuge dool down and what you can do minutely is quite a bit lower (4 x fel rush + fury + eye beam) and requires you to blow your only def cd (2 extra fel rushes when activating blur on the artifact).
Overall both are AoE monsters. Burst AoE I would really give to DH if we can talk about 1 min intervals. If we have smaller intervals or simply quite long AoE windows, especially once were you can't use eye beam every time then unholy wins more consistently as epidemic takes a maximum of 30 sec to get all 3 charges back and DnD will also be back up again.