The folks at DisplayMate took the photo above which shows the Apple Watch Sport (left) compared to the regular Apple Watch (right). The photo was taken in 2,000 lux uniform ambient light and as you can see, the Apple Watch Sport is brighter and easier to read compared to the Apple Watch.
According to DisplayMate, this is because light has to pass through the crystal twice, thus reducing its brightness. Dr. Raymond M. Soneira writes, “Minimizing screen reflections is especially important for displays, and sapphire has almost double (191%) the Reflectance of glass.” However he notes that this is only an issue when using the watch under bright sunlight.
Soneira states that if you were to use in the dark, then the differences shouldn’t really matter or even come into play. It’s actually interesting that the cheaper Apple Watch Sport is more legible than its more expensive and luxurious sibling, so perhaps that might be worth considering if you thought that having a sapphire display was an added advantage (perhaps in durability).