Damaging a Rust Monster w/ a Magical Weapon

[dcollins]

Rust (Ex): A rust monster that makes a successful touch attack with its antennae causes the target metal to corrode, falling to pieces and becoming useless immediately... DC 17 Reflex save...

A metal weapon that deals damage to a rust monster corrodes immediately.

Notice that there's basically no difference in the description of the hit-by or hit-against cases. They both say that the corrosion happens "immediately" (i.e., instantaneously).

It's pretty clear the save applies in any case, since the description is effectively identical.