Why do you like -5/20% vs -2(Stance/Enchantment)? The -2 is better assuming you can always keep your Stance/Enchantment up. Assuming you're using that modifier you'd most likely have a build that always has a Stance/Enchantment around.
I use the shield occasionally with my Ele (very occasionally in PvE), and the -2 is practically nothing for my type of caster armor, it doesn't matter what enemy hit me because they will always damage me a lot, so I prefer having +20 extra armor to reach 90 (+10 of the shield itself and +10 of the inscription) or trying my luck with the -5. Both options without using a skill that gives armor, only using weapons I mean. Also sometimes I use a sword with armor +5 or +7.