The obvious advantage of face shield sports sunglasses is that they provide a wider and unobstructed view. This is because the single-visor lenses are not blocked by the bridge of the nose in dual-lens sports sunglasses. This feature is useful for cyclists who often like to see their instruments on the handlebars and also like to see the road without moving their head. This is a favorite/must-have for triathletes on a triathlon bike.

Sunglasses have larger overall lenses than single-lens sunglasses, which means they filter more light and provide better UV protection and impact protection. If you opt for rimless face shield sports sunglasses, they are likely to be much lighter than rimmed sunglasses. This is great for long workouts/races. In theory, the optical clarity of face-shield sports sunglasses shouldn't be as good as dual-lens sports sunglasses because the lenses have to be curved over a wider distance and in two planes.

It is more difficult to develop a production method for the larger single polycarbonate sheet lens material and therefore may be of lower quality. For this reason, I recommend never buying a cheap pair of face mask sports sunglasses, as the optical clarity can be poor and they can be made of defective or reused polycarbonate sheet lens material.

Which good factories dumped on the market and then the material was used by notorious lens makers. The main advantage of dual-lens sports sunglasses is that optical clarity should be better for not-so-great sunglasses because it's easier to make smaller polycarbonate lenses. The curvature of the lens is less, resulting in less chance of optical distortion. Second, two separate lenses are generally stronger, especially for impact damage. This can be a useful advantage if cyclists and mountain bikers expect objects to hit the lenses.