the color is just a matter of stretch (more obvious in the linearly-scaled data, less visible in the log-scaled data). Please make sure to apply the same minimum and maximum values in the Color Manipulation tab to make the images visually comparable.
If conversion to dB makes sense or not depends on the type of information you want to identify. If you want to separate urban from non-urban land, the linear scaling gives you better contrasts. If you want to investigate changes within the built-up areas, the dB-scaled data might reveal more details.