We present the first scattered light images of debris disks around a K star (HD 53143) and an F star (HD 139664) using the coronagraphic mode of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). With ages of 0.3-1 Gyr, these are among the oldest optically detected debris disks. HD 53143, viewed ~45° from edge-on, does not show radial variation in disk structure and has a width >55 AU. HD 139664 is seen close to edge-on and has a beltlike morphology with a dust peak 83 AU from the star and a distinct outer boundary at 109 AU. We discuss evidence for significant diversity in the radial architecture of debris disks that appears unconnected to stellar spectral type or age. HD 139664 and possibly the solar system belong in a category of narrow belts 20-30 AU wide. HD 53143 represents a class of wide-disk architecture with a characteristic width >50 AU.