If you ride in sunny Southern California, a darker lens will leave you more comfortable, but if your trails snake through the trees with a dense overhead canopy, light or even a clear lens will be the go-to.
Lots of companies have their own versions of contrast-boosting technology, and they all work to varying degrees.
Depending on the lens, some brands will add hydrophobic and anti-fog coatings to their lenses, and these can be a godsend in wet weather or riding behind your roots-riding mate.