Playing double mint on 3 is also fine, but if you play it at 4, you won't likely get much profit out from it since games usually end fast.
You play double mint in a way that it forces your opponent to address it. It is like forcing your opponent to spend brains on it instead of on your strong plants.
There is a rather famous term in Magic about this general topic: BUT IT DIES TO DOOMBLADE!
And trust me: you rather want a Game with good removal and proper answers than a game where players just fling legendaries at each other because they have overhelming stats.
That is why I as a fan of efficient control decks have nightmares of wanna-be-hero :/
both are great cards - your opponent MUST defeat either card on the following two turns or they are screwed. they can't ignore it and leave it for long, allowing you to pull off something else entirely.
