All other things being equal, how much of an effective bonus is wearing a piece of armor you have the appropriate armor use skill, towards not?

Say I have a piece of leather that has AC X, and I have light armor use. I also happen to have a piece of metal platemail, that in all other things is identical, except I don't have metal armor use, but it has a higher AC. How significant should I assume the effective use to be? Better to wear the metal armor only if it's significantly higher AC, or will it almost certainly be better to wear the light armor?

Obviously this comes into play with a lot of rogues, and more melee-oriented classes in regards to the significant amount of dragonscale armor that's about. Wondering if protection-wise, the dragonscale's ever worth it. Damn the secretive dragon armor use guildmasters!