If i were aiming for barrier, i would use smart coat instead since it give me more SPR in general. Not to mention it give use magic resistance, which give protection against all mag damage ( including frost blade), while armor of light can only protect you from mag attack
Ah sorry actually both of them can only protect you from magic type attack ( frost blade aren't affected), but the point is it give you more SPR ( around 21, if i remember correctly).
Barrier. If you plan to use WoL or sasayaki as your main tank.
Reasons:
1. Harder to find an armor with magic resist and decent spr
2. Lots of heavy armor have better def bonus, with phy atk type resist, from future events as well. So this one is less valuable than other armors
The problem with your statement is that there are units, Warrior of Light included, that cannot equip Smart Coat. Barrier-type AoL is currently the best magic-resistant armor they've got.