You missed a big plus of rust. You can cast it preemptively and flip it when needed. If you need to use an acid ball your opponent probably benefited from his armor against at least one attack which means lost damage.
Not saying you need rust, just saying it's the biggest pro on rusty in my book.
Something to keep in mind is that is also a disadvantage of Rust for 3 reasons.
1.If the opponent never casts an armor source the action and mana are wasted.
2. The facedown enchantment is vulnerable to dispelling before it ever gets flipped meaning you may not get use out of it in a game.
3. The Acid Ball still statistically does at least 1 critical damage on cast meaning it is forward pressure AND armor degradation where Rust is only armor degradation.
In a drawn out battle with extra unneeded actions or if you have good deck synergy Rust can have more going for it. In a damage race however Acid Ball wins out (Statistically) hands down. It also still functions as a cheap attack spell doing 1-6 damage (Statistically 4) if the opponent has no armor to speak of. Every damage counts.