Those on the south side should save energy and allow as much solar energy as possible to pass through. Obviously, the entire window R value is a better indicator of performance. Don't be confused if you see a U value alongside the familiar R rating. They are different sides of the same coin. R values measure resistance to heat transfer - the higher the better.
U values measure heat transfer - the lower the better.
To keep everything straight, convert U values to R values by dividing the U value into 1. This rating tells how much solar energy a window captures. A single pane of glass has a shading coefficient of 1. A blocked window would be rated at 0. So, if reducing cooling costs is important, pick a window that has a low shading coefficient.