Solid ceramic logs are an older design than newer ceramic fiber logs. Solid logs, aren't designed to directly touch the flame, or they would create too much soot. Because solid logs are hand-painted instead of formed, they tend to have more detailing.
With fiber logs, the flames often hit the surface, and that causes the logs to glow. This results in a more natural appearance. Hearth.com's employees have tested ceramic fiber logs and many of them have lasted three or four years with little or no problems.
According to the Minnesota Blue Flame Gas Association, most homeowners choose ceramic logs for aesthetic reasons, and not when heating efficiency is a priority. The organization recommends a fireplace insert if efficiency is a primary concern.