Lens material is more important while purchasing goggles. It can be a pretty tough decision when you look further into it.
Polycarbonate lenses that resists cracking and is strong. You can get this in a variety of colours and can also feature anti-fog properties and UV protection. You'll find most lenses are made with a plastic but there can be a huge difference in quality and quite often it's a case of you get what you pay for.
Optical Grade lenses, a high grade plastic that's used in the construction of prescription glasses. It's lightweight, scratch resistant and has less distortion than a cheaper lens.
Anti-fog coating, a thin layer of anti-fog treatment that is applied to the lens. It does wear away over time and will need to be reapplied with one of the anti-fog drops or sprays that are available to buy separately.
The lens is very important, as the main purpose of goggles is to put a lens in front of your eyes. On most goggles the lens is actually made of 2 separate lenses put together. Between the two lenses is a foam divider creating an air gap between the layers. This insulates your face more and also stops the inside lens becoming as cold which makes the goggles more resistant to steaming up. Lenses are normally made from a polycarbonate or Trivex plastic, which makes them more impact resistant and less likely to shatter than glass or normal plastics.