It's important to know how the two stats work to make a decision. Elemental Resistances are just straight up damage shaving. For example, if you have 100 points of Fire Resist, it'll shave 100 damage off any incoming Fire attacks. mDef, on the other hand, is run through a formula along with the attacker's mAtk to determine a percentage to modify the base damage by. For example, if you have the same exact mDef as the attacker's mAtk, the spell does 100% of base damage. From all accounts, having more mDef than their mAtk is a straight linear reduction; for example if their mAtk is half your mDef, they do 50% base damage (half damage). But if their mAtk is greater than your mDef, it starts to get into diminishing returns; twice as much mAtk yields less than double base damage.
mDef is provided directly from Wisdom, but few short of healers are going to have that in more than negligible supply. Armor also provides it, but, again, anything other than Cloth is going to provide a negligible amount. I'd almost say the difference is marginal enough that it shouldn't be the deciding factor on picking a piece; but if that's really all it comes down to, I'd say go for the mDef as default unless the eResist is particularly high and you're facing rapid-fire spells of that element (since the shaving happens on a "per-hit" basis).
