If all else fails, check with "Car Talk"! Look HERE.
As you suggest in your post, when you tilt it up (as it would be when in place on the car) or in direct sunlight, it is better. As it says in the article that Mike posted and the one from "Car Talk", that's because of the polarization of light waves.
I've got a fantastic pair of prescription sunglasses, which I carry around in my Camaro because of glare. They are a yellowish-orange shade, that really reduces glare and actually makes everything look brighter on shady days. But, when I look back out the side windows at a certain angle, the "diamonds" in the glass are more pronounced.
Sorry, I've still got original side windows, so I do not know much about replacement glass brands...