Your reliance solely on ERA implies a faulty assumption: that a pitcher is more or less consistent the entire year.
Fictional Pitcher A: 20-8, 3.50 ERA, had a stretch where he couldn't command the fastball, but he's an ace, so his manager left him in when he was getting rocked, trusting him to get out of it. He gave up 30 runs over 5 starts, and lost them all. But the rest of the year he went 20-3 with a 1.60 ERA. Those 5 starts really rocked his ERA, but it was still a fantastic year.
Fictional Pitcher B: 14-11, 3.20 ERA, had a really nice season, consistently pitching well in games. Being a rookie, the couple of times that he got into trouble early, the manager pulled him to limit the damage, and it saved his ERA.
I contend that Pitcher A's season was superior. If you take Pitcher B in this situation, then I want you to go GM the other teams in the division, because we will beat you.