Quartz
Hero
It is much, much better. The limiting factor of Unarmored Defense is that to get a decent AC out of it you have to have a very good score in two Abilities.
I disagree. It's not limiting if you're going to have good scores in those abilities anyway. Consider a 4th level barbarian with Str 16 Dex 16 and Con 16: she can have AC 18 with a shield and no armour. which is only two behind the fighter's AC 20 with plate and shield. If you allow the fighter an unarmoured defence feat then he drops two stat points for an initial +2 to AC, rising to +6 at high levels, but they still need a good Dex for a top AC and they still need a good Con for the HPs.
But it's about the play-style you want: if you don't want fighter-types clunking around in plate then you need an alternative, and using the Proficiency Bonus fits very well.