Depends on what you do and how you treat your glasses.
If you tend to get scratched lenses, I highly recommend getting a pair of Smith sunglasses. They use glass lenses which are infinitely more scratch resistant than polycarbonate and have better optics. I had my Smiths for over a year (a point after which most of my shades get MAULED) and they ended up falling victim to a broken frame, but the lenses are just like new.