[lch]

Marble Armor: Head prot 15, Body prot 24 => ~ 23 prot
Robe of Invincibility: Head prot 25, Body prot 25 => 25 prot

4 times as many gems for as little difference as that? Marble armor is as cheap as a Black Steel Full Plate, the robe is more expensive than Aseftik's armor and the Monolith armor. You have fun with your robe while I'll kit out my four SCs... The Marble armor is just plain easier to forge and thus mass-produce, too. Combine with Frost Brand if possible.