The Apple Watch Sport comes with Ion-X glass, where as the Apple Watch and Watch Edition come with sapphire glass.
Sapphire glass ins incredibly tough and scratch resistant and we have already seen a test of the glass.
The video below from the guys over at Ubnox Therapy shows the Ion-X glass which is used in the Apple Watch Sport being tested for scratch resistance, the glass is not as tough as sapphire glass, have a look at the video to see how it stands up.
The Ion-X glass was then tested with sandpaper and this managed to scratch the glass considerably, if this was done on sapphire glass the glass would be much more resistant to scratching with the sandpaper.
Source Unbox Therapy / YouTube