Due to a bug, this makes the ethereal 50% defense bonus apply twice, since the ethereal 50% mulitplier after socketing uses the shield's original total defense value instead of its base defense value to calculate the shield's new defense. etheral for starters as it self repairs and etheral normally has better stats and damage but at a cost because it loses durability which is why that self repair is soooo important and to add to that the stat requirements for using etheral are less than if it was non-etheral. Out of curiosity, I guess finding one is quite difficult and would required Hell runs stripped of my MF?
When it comes to which base shield, it depends on preference. All elite paladin shields have the same average damage. as a Mod so ideally you should find an Ethereal one for better stats and low requirements and of course always find the highest base resistances you can (like 35+).