The f/1.8 aperture will be best for night time, when there is little light.
There seems two philosophies about the best carry-one-lens-and-walk-around-all-day lens.
Some go for the maximum zoom range, like 18-200 mm, to be ready for any situation.
Others prefer the better quality (sharpness, distortions, etc) of a more moderate zoom, like the 16-85 mm for example. The extreme zoom ranges have a cost.