Regardless, I said 28 AC is overkill and suggested to go with another magic item in that "slot," not to take it.
That depends on how you play your Wizard. Mine is going to spell snipe at range and isn't going to be in many situations where I would have to use Shield every turn to avoid being hit. Nor would I be in a position to utilize AoOs very much since that requires being right next to the fighting.
The 25 AC Wizard means you are going to have to sink more (or different kinds of) monster resources into damaging him than otherwise be necessary. It is an easy-to-obtain defensive measure meant address a normal Wizard weakness and to frustrate the enemy, not end encounters.
I guess at L18 I'm not as concerned about a 25 AC character in terms of monsters & other classes. A Monk can hit 23 and spend a single Ki point to grant disadvantage with a single magic item (20 WIS & 20 DEX is definitely reachable, with 4 +1 stat boosts by L16 and a single magic item); A eldritch knight can do something similar just based on the class. 25 might be high, but if the dragon is any indication, there are a lot of non-AC attacks that would threaten someone.
Also, I would be careful about assumptions on how magic items are going to work, especially with regards to "slots".
