On paper in terms of overall (long term) mitigation, Allagan/Holy shields are better. I am too lazy to find the maths/figures but the benefit is less than 1% difference in overall mitigation.So, it's not a class I'm going to be using often, but here goes.
Onion Shield vs Allagan Round Shield.
Which is better, GO!
In practice I vastly prefer Onion. Even with slightly lower overall mitigation, you will end up mitigating more total hits. This is more important to me, as I prefer smoothed out damage. Most of the time the only thing that should kill a tank is burst, and I will take a higher chance to reduce that burst over a lower chance but slightly higher reduction.