The price difference for residents vs. non-residents is most unfortunatatly true. Yes, as has been noted in many threads a non-resident may in practice book and ticket a resident-only fare but in that case you are rolling the dice.

While its also true that enforcement of this rule, at least with AR, is in my experience rather lax, if you have an unlucky day you may have issues at boarding. I dont know how tough LAN is with enforcement of similar rules.

On a final note, phreegreens is correct is saying that whenever possible, take LAN over AR for domestic flights in Argentina.