The difference between them is statistically undetectable in any realistic MLB season or seasons. You'd have to create some alternative reality where you can get the very similar conditions for each catcher for a really significant period of time.
I don't fully buy into pitch framing stats for similar reasons, but at least pitch framing is only trying to measure the catcher's role on the smallest building block of pitching, whether a borderline pitch should've been and was called a strike or ball. ERA is the end result, a combination of many things with heavy influence on outside factors. Much harder to accurately isolate the catcher's influence.