Spherical lenses provide a larger field of view and reduce distortion and glare, especially under artificial lighting (night skiing) as opposed to cylindrical (flat) lenses. The only real downside we see to spherical lenses is the extra cost, and I suppose they can be harder to take care of since the curvature of the lens actually extends past the frame of the goggle. So if you lay the goggle down flat on a table, a spherical lens will rub where as most cylindrical lenses will not touch the table. Flat lenses are also typically more flexible than spherical lenses, making them more durable.