You're right, I didnt account for ghoul damage. Again picking a random fight (
https://www.warcraftlogs.com/reports...done&source=10) of exactly five minutes we see Army of the Dead being 6.5% of damage. That fight had 4 Apocalypse casts (since its a top 100 parse lets assume he summoned 6 ghouls each time).
So we have 8 * 35 seconds = 280 seconds of ghoul uptime from AotD (assuming it was precast and he didnt get full duration) and 4 * 6 * 15 = 360 seconds of ghoul uptime from Apocalypse.
So a total of 640 seconds of ghoul uptime = 6.5% of damage. For easy rounding we can say each 10 seconds of ghoul uptime adds 0.1% to total damage. Since a lot of that uptime is during heroism probably a number like 12 seconds of ghoul uptime later in the fight to add 0.1% is more reasonable. One extra Army of the Dead in the duration would be 2.7% extra damage, two would be 5.4% total damage (on top of the 5.5% I estimated earlier) which is quite a lot.
The final thing I didn't take account of is the rune cost. Spending 9 runes on Army of the Dead (I'm guessing it won't be precast since you would waste half the damage increase) would mean (on that parse) losing about 10% of his clawing shadow damage and presumably about 10% of the festering wounds damage too at a total cost of 3% damage lost.
So we end up with 5.5% increase from previous estimate plus 5.4% for ghouls minus 3% for the runes you lose out on for a total of about 7.9% increased damage.
At this point the big question is how quick you can recast AotD with the set bonus. If the timing of the fight works out such that you could cast it 3 times in that fight it would be pretty strong, if you can only get off one extra cast after the initial the value would drop quite substantially to more like 5%.
That would mean the you would probably see which was the stronger tier bonus vary based on fight length. If the fight is just the right length to squeeze in an extra army the Unholy would be stronger, if the fight was just the wrong length of time to do that then Frost would be stronger.
The two tier bonuses look (from my admittedly very error prone and very likely to have missed something perspective) to be reasonably close. Which ends up the stronger spec overall will probably come down to which is the stronger spec without the tier bonuses.
I'd be wary of anyone claiming one spec or the other will win out though. I'm sure I'm not the only one who remembers how certain everyone was that Unholy was going to be the stronger spec going into Nighthold (and the various "this is the death of frost" posts quite similar to those we see now) after the last round of class balances and even months later after some hotfix buffs its still lagging behind by an appreciable amount. It's fun to guess and speculate but nobody knows at this point.
P.S. Just for reference, based on running two sims one swapping out Dreadwyrm Crown for an equal ilvl Eventide Casque and thus keeping very much the same stats but losing the 4 piece bonus the current 4 piece bonus is worth about 3.8% increased damage to Unholy so the numbers above would put the set pretty much in line with the current set for power.