Parrying aside (which is devalued by the higher block rate anyway), the shields are within half a percentage point of each other with regards to mitigation. Just multiply your overall block rate with your overall block strength.
Holy/Allagan: 0.28 * 0.28 = 7.84%
Onion: 0.39 * 0.19 = 7.41%
I lean towards the Onion Shield because the higher block rate (nearly 40%) means it's more likely to proc on stuff I want it to, but in the grand scheme of things I doubt it will really make any difference beyond aesthetics.
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