And the armours of Alteration.
Since shield is a channelled effect, I think it will provide a greater deal of protection than Mage Armour/Ward spells. The drawback of it being a heavy magicka drain and not being able to dual cast offensive whilst using it.
Most likely.
Also, we know that restoration wards are more general-purpose while alteration magic is more specialized:
Wards increase armor rating AND protect against magical damage.
Alteration armors only increase armor rating
Alteration shields only protect against magic (or melee, but not both at the same time).
Furthermore, wards have the advantage of protecting against any type of magic and, with a perk, to absorb the magic cast at you to refill your magicka bar. Compared to that, alteration elemental shields may only be protecting against one type (fire shield, shock shield...). Of course, perhaps there is an all-encompassing elemental shield under alteration but at least in oblivion there wasn't.