Back in "Beta", the idea was that every level of armor added more protection, and it appeared as such in my experimentation.
My 'OPINION' of how it seemed to work:
Military added 50% armor protection.
Mirrored added 25% armor protection, and a 50% boost against laser attacks.
Reactive added 25% armor protection, and a 50% boost against ballistic attacks.
I.e. You can take more shots, but every shot you take still hurts; it just hurts less if you have the matching resistance.
(AGAIN, MY OPINION OF MY TEST RESULTS)
However, I've upgraded to Mirrored in Live, and it really seemed to be no added base armor protection, but more /RESISTANCE/ against Laser attacks, which the net gains (To me) didn't seem that beneficial.
I went back to Military grade and felt more protected. (And saved a few spacebucks)
Again, this is just my opinion of use, and it was tried on both an Eagle and a Cobra, when I have been running without armor for weeks.
I can tell you, that with both Mirrored and No Armor, if you land hard / graze the landing ramp on touch down, you'll take between 2% and 8% damage. When I went to 'Military grade', these kinds of 'Grazes' didn't even register a single % damage. Plus Military Grade seems to be more damage resistant. (I am curious how Ballistic protection would do against grazes)
It seems the 'Base Armor Increase' that I perceived in Beta with Mirrored/Composite no longer exists.
Again, these are just my opinions. Hopefully there is more factual information out there. (A 'Jane's Guide to Elite: Dangerous Ships and Weapons' would be *AWESOMELY Appreciated*.)