Comparison between Holy/Allagan, Onion, and Direwolf:
Holy/Allagan: rate - 181, strength - 181. Overall block rate of 28%, mitigation 28%
Onion: rate - 295, strength - 92. Overall block rate 39%, mitigation 19%
Direwolf: rate 108, strength - 288. Overall block rate 20%, mitigation 38%
All those values are rounded down because I don't trust SE to round in the player's favour, ever.
Over 1000 hits for 500 each:
Holy/Allagan: 280 blocks, 140 damage blocked on each hit, 39200 blocked total
Onion: 390 blocks, 95 damage blocked on each hit, 37050 blocked total
Direwolf: 200 blocks, 190 damage blocked on each hit, 38000 blocked total
Personally, I lean towards the Onion shield just because it's theoretically more likely to proc on hits you'd want it to (Death Sentence), but they're close enough together that I'd just use whichever you like the look of best. Personally, I have zero interest in PVP.