Keep in mind when calculating the difference between UAring and the SS that UA also benefits from Pillar of Frost and Unholy Stength.
The same applies to Unholy via the 20% straight buff + Unholy Str. (25% next patch I think)
Average Strength bonuses for Frost/Unholy DKs
10% flat (Kings/Plate)
6.6% from Pillar of Frost
12.2% from UnStr
~28.8% extra for Frost
30% flat (Kings, Plate, Spec)
12.2% from UnStr
42.2% extra for Unholy
Note: When calculating AP, use a 10% flat buff, 20% w/ upcoming patch. Also, figures use 33% (max) uptime of PoF, and 75% (No ICD) uptime of Unholy Strength (Fallen Crusader).
Remember that the % buffs (plate & kings + others) are applied multiplicatively, so we actually get a little more from them. Here's some math on the ring, assuming the above values:
36 *1.05*1.05 = 39.69 (kings + plate spec)
39.69*1.0667 = 42.34 (the above with PoF averaged)
42.34*1.122 = 47.50 (the above with FC averaged)
47.5 str translates to 95 AP for a plate user (assuming the .5 str isn't truncated, but it only accounts for 1 AP in this calculation). All AP buffs affect both STR and AP in the same way, so calculation for them is irrelevent. But, 95 AP is around 50% more than almost all of the epic tbc rings that are available, which
should provide more of a dps boost than the better secondary stat allocation from other rings. Considering also that this ring has 1.36% crit on it, vs the 1.97% haste on the Band of devastation. For calculation purposes, we can probably assume that the 28 crit rating is around equal to 2/3 of the available haste rating on the trash drop. Which does bring these two much closer together. Haste is much better for frost than crit is because of our talents, which is why I've made this assumption
However, being that str provides our ghoul with much more of a boost than haste does, it's safe to assume that for PvE, on a boss where you'll experience high uptime of both RotFC and PoF, aswell as 100% ghoul activity (it spends its entire lifespan hitting the boss), that the Unstoppable Aggressor's Ring is the better solution.
There, some math for ya
EDIT: Also, for the Blood tank chardev, you could gain a little more stamina (if that is the aim, ignoring stats such as dodge or expertise for huge stamina gains) from
http://ptr.wowhead.com/item=35133 since the agility doesn't do much for tanking, it's 6 more stamina for doing nothing.