Agree - anything stopping some of the sun's rays entering the house is best. But I have tinted windows and use thick curtains. The sun penetrating a room will fade fabrics - so the curtains may be a cheaper option to replace occasionally. Curtains can be black-out types and can even be doubled. I have no figures to prove this but it seems intuitive that the sun warming the curtains and the heat from the curtains then slowly dispersing into the room must be better than the sun's rays directly entering the room and immediately heating up the air molecules therein.