Mike,
...the polarized lenses protect you from the UV rays as well as other things..Polarized lenses contain a special filter that block intensely reflected light, reducing glare.
Mirrored lenses just reflect the light away but don't provide much protection.
They do however, make mirrored-polarized lenses which do both, plus the added feature of not letting the women on the beach know you're looking at them....especially when they're adjusting their straps...
Here's a good
article if you enjoy reading