I don't think Miley is a 3.00 ERA pitcher. I don't think he's gonna be a major asset, but I think he'll be a capable back-of-rotation piece.
I think Miranda will struggle to remain in the majors. If he establishes himself as a useful starter, then he's certainly more valuable than Miley given cost.
It's not so much about establishing yourself. It's about getting what you can out of someone before they start to suck and identifying when they will and getting rid of them or avoiding signing them when they show signs of sucking.
Pretty much all of Duquette's SP acquisitions have been terrible.