iPhoto used to do this, where you choose two similar pictures, and are able to compare them; either side-by-side, or going back and forth though them with a flick of the thumb, like you would two photos in iOS’s Photos app, where they happen to be next to each other. My workaround for now is choosing two photos and placing them in an album and comparing them. Kinda convoluted, so I’m hoping for an easier method.
I flag or favorite them so that they appear next to each other in Flagged/Favorites photos. You can then use the film strip on the edit screen, tapping on each photo in turn. I find this makes any differences, between the two photos immediately apparent in the edit view.