I've seen these questions come up a few times and wondered about them myself, so i did some quick math.

- With Damage Shield, is 18 SBV the better shield enchant for AE tanking, or should we still go with good ol' Shield Spike?
- Which is better for AE threat, Block Value or Attack Power?


assumptions:
- Shield Mastery
- SBV meta gem
- mob attack speed: 2.0
- not accounting for Incite

Blk: Block Chance
Av: Avoidance
SBV: Block Value
AP: Attack Power

Shield Spike vs. 18 SBV

Spike: 32 damage per blocked attack
Threat = 32*Blk
TPS = (32/2)*Blk = 16*Blk

18 SBV: Damage Shield deals 0.2*SBV damage per not avoided attack
Threat = 0.2*1.35*18*(1-Av) = 4.86*(1-Av)
with (improved) SB uptime: (0.75*4.86+0.25*4.86*2)*(1-Av) = 6.075*(1-Av)
TPS = (6.075/2)*(1-Av) = 3.0375*(1-Av)

equation:
16*Blk = 3.0375*(1-Av)
=> (16/3.0375)*Blk = (1-Av)
~> 5.3*Blk = (1-Av)

This means that for 18 SBV to be better, your chance to be hit or block (1-Avoidance) has to be 5.3 times higher than your chance to block, which will not happen if you're not wearing nothing but a purple lunar festival dress.
Stay with your Shield Spike.


Shield Block Value vs. Attack Power

SBV translates into AE threat via Damage Shield:
Threat = 0.2*1.35*(1-Av)*SBV = 0.27*(1-Av)*SBV
with (improved) SB uptime: (0.75*0.27+0.25*0.27*2)*(1-Av)*SBV = 0.3375*(1-Av)*SBV
TPS = (0.3375/2)*(1-Av)*SBV = 0.16875*(1-Av)*SBV)


Attack Power translates into AE treat via Thunder Clap and Shockwave:

TC:
Threat = 1.3*1.85*0.12*AP = 0.2886*AP
TPS = (0.2886/6)*AP = 0.0481*AP

SW:
Threat = 0.75*AP
TPS = (0.75/20)*AP = 0.0375*AP

TC+SW_TPS = (0.0481+0.0375)*AP = 0.0856*AP


equation:
0.16875*(1-Av)*SBV = 0.0856*AP
~> 2*(1-Av)*SBV = AP

This means that at 50% Avoidance, 1 SBV and 1 AP have roughly the same value for AE threat. More avoidance favors AP, less avoidance SBV. 1 Strength would be worth 2.5 AP or 2.5 SBV at 50%.
Incite does further increase AP's value.
So item point wise, value AP over SBV, and Strength is of course king anyway.