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:
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.