More what I meant was that Plate+Shield is 1 point better (equal to Mage Armor+Dex+Shield), and thus heavy+shield is "the best armor" while medium+shield is slightly less. (Plus, Mage Armor is a non-negligible cost in the early stages of the game, though I guess you could probably make do with Light Armor at first and switch to Mage Armor once you had a comfortable number of 1st-level slots...)
Definitely worth remembering the casting/shield thing though. Clerics can get around it with the Holy Symbol, as you (implicitly) noted, but Sorcerers can't. Unless they focus almost totally on pre-combat buffs and post-combat healing, the FS Sorc has a very strong incentive to not use (or frequently stow and re-equip) a shield, and thus have only 17 or 18 armor. This is of course still good, but it's definitely a step down from "best," and that's before accounting for Fighting Styles and other sources of AC that go above and beyond armor and shield/Mage Armor and shield.