I think CON is better on wizards. Both experience, and other posters' guides to wizards, suggest that maintaining concentration is more important than have a higher AC against attacks that one shouldn't be in the way of anyway.
Agree partially, it requires the wizard saves his standard reaction to cast shield and has this spell available to him. It gives you a boost in HP for melee situations with though hitters, aka mobs with high + to hit.
Otoh you got only one reaction, so a horde of mobs is not your friend with this build, as are most damaging area effects like enemy fireball dragonbreath etc.
So here Dex saves the day, for its AC boost,
E.g. If you use your reaction for shield, you cannot use it for counterspell anymore. So that's a tough choice.