Why should I care whether armor has less mass vs is more effective? Less mass IMPLIES more effective as I can simply add more armor modules.
The specializations all make me go... meh. I'll take the mass option.
Here's a better idea:
1) Armor specialization:
a. Explosive armor (armor modules now provide +2 missile defense)
b. Shielded armor (armor modules now provide +2 shield defense)
c. Armor takes less mass.
Now that's a choice I'd think about.
This has the added benefit of letting people get a little defense in other categories without spreading out their tech.