The difference is in how the regular path (exercised most of the time) vs an edge case when there is a conflict (typically in larger clusters with a pathological access pattern) works. With TrueTime, the latency is always 7ms and no issues in the pathological cases. With HLC, the latency is lower in most cases, but high in the pathological cases (when it can be 500ms), but these should not matter for many use cases.