One thing I'm surprised no one has brought up is the value of having both shield and mage armor. It is a heavy slot cost, but in my experience once you get past the first few levels a 3 point increase in armor makes shield much more effective. It's inevitable that things will get past the front line once in a while, and shield taking you up to ac 17-18 isn't enough to stop most enemies once you pass level 5 because they need a decent chance to hit the front line. Add to that the diminishing value of level 1 slots for other purposes, and its less a choice of mage armor or utility/offense and more a choice of mage armor or 1 more shield/absorb elements. I'd much rather have shield bring me up to ac 20-21 than only be useful against enemies that roll low. Also keep in mind that if your dm is strict about the rules, he will only tell you he hit rather than what he rolled, so with mage armor up, your remaining shields will be much less likely to be wasted.
I think this is one of the disputed points.
The value of Shield varies with whether the GM chooses to roll in front of players or behind screen.
Note - both are legal choices - no rule dictates it.
JEC has said in
his games if someone has abilities that are like "decide after roll but before resukt" he gives them the roll since that is part of the thing of the ability - after the roll but before is meaningless if you dont show the roll.
Shield simply says after hit, not after roll and JEC has confirmed that RAW seeing the roll is not required but i have never seen a gm making open rolls pull attacks vs shield users behind the screen. The rule for shield doesnt require either hidden roll or revealed roll.
I dont think i have even seen them not inform the shield user on the quality of the hit like they normally would.
So, in my play and play i have seen, the number of times shield did not cancel a hit was nil.
I would say flat out, if your gm hides attack rolls from shield users but shows them to say folks with say choices like Combat Inspiration or Cutting Words to see the roll before deciding - then drop shield as an option. If the GM doesnt show any of those rolls, you got bigger issues to worry about.
My analysis based on my gameplay experience plus math is stated in the above posts. But in summary...
The frequency of hits to misses by Mage Armor for tier-1 mage means by the time i am hit enough to make Mage Armor better than Shield my low level mages are screwed on the damage anyway.
Mage Armor only beats Shields in losing anyway situations.
Having shield as an emergency use when it gets desperate but mostly planning on using those slots for much better spells like Fog, Image, Charm etc that can affect .more thab just "hits" has proven to be much more viable a strategy.
That may mean, yes, an "evil GM" has a complex situation etc and i have AC 12 vs AC 15 but in my experience those kinds of battles and setups are even better cases for having those other spells burning my slots. Mage Armor wont win us that fight as much as the other options will.
By tier-2 and beyond, other options start looking better anyway.