The change to a swipe might reduce the accidental fat finger, but discovery is a nightmare for both the gesture itself and application specific behaviors for the back event.
I prefer an application button in both cases, especially because it makes it clear that this is something that will only effect the app, and will not have a weird OS interaction, taking me out of the app.
The underlying problem that makes them a bad interaction paradigm, in my opinion, is that they have no affordances whatsoever. You just can't tell by looking at the UI that swiping left/right/up/down is going to have an effect.
All swiping should work that way, and Apple is emphasizing the importance of "physical" dynamics in iOS 7.
I think another possible way of indicating the presence of a gesture might be through clever use of shading - for example, a subtle gradient on the edge of the screen where you can swipe, giving you the impression that there's something "just off the screen". I haven't actually seen this in an app yet - not sure why this isn't done more, especially for the Pull To Refresh trick.