If you make a Curves adjustment using the Luminosity setting, the end results will generally be less vibrant, but often more subtle and naturalistic. The end result that you are seeking will dictate whether you use this setting or RGB.
As mentioned before, we are going to add a simple 'S' curve to boost contrast. With the drop down set to Luminosity, click on the diagonal line towards the right end and drag it upwards. Then move to the left hand end and click and drag the line down. You will see that the straight line is now curved into a rough 'S' shape. You'll also see that the image now has a bit more contrast giving it a lift over the original. On the next page we'll try using the RGB setting.