the ring gives a heavy shield with:
+2 AC
no offhand (heavy shield does not allow) -> no casting, no weapon
free action to de-/activate
no armor cheack penalty, no arcane spell failure
8.500 gold
a mithral buckler +1 gives
+2 AC
offhand (but loses AC on use) -> spell casting possible
no armor cheack penalty, no arcane spell failure
upgradeable to +10 (+11 AC)
2.015 gold
in my opinion that ring is useless :D am i right ?