currently ilvl determines the stat-caps for an item, and the highest level materia that can be melded on to it.
Armor type determines it's Def/Mdef values.

Items that share an ilvl tier will have different stats on them (generally)
Str / Vit / Parry / Determination
or
Str / Vit / Accuracy / Parry

things like that, so your choices across a shared ilvl on items is not based on primary stat/def but on which secondary stats will help you the most.