An eph-type explosion originates at the center of the larger body. A central explosion of a planet-sized body allows many fragments originating far from the center to travel along nearly parallel trajectories with similar speeds that diverge much more slowly. In an impact, small fragments tend to leave first and travel fastest, with large fragments lagging behind. Both mutual separation and the radius of the gravitational sphere of influence of each increase linearly with distance traveled away from the impact site. This accelerates the speed of increase of their sphere of influence above linear with distance traveled, and allows them to capture nearby debris.
Impacts are almost exclusively destructive.
A planetary explosion propels considerable trailing material into the larger fragments that left first, causing them to accrete some additional mass, and enlarge their spheres of influence faster, on the way out. Gravitational captures from a collision event are nearly impossible. When ejected by an impact, the spheres of influence grow linearly with distance from the center of the parent body. But the actual separation grows linearly with distance from the impact site near the surface of the parent body. Since the latter is always greater than or equal to the former, no two co-moving fragments can ever get inside each other's spheres of influence.