Well, that is going to depend. If you want more light, better airflow and visibility, you should go with 80%. If you want more heat, glare, and UV light reduction and energy efficiency you should go with the 90%. You can also mix and match your protection levels. Put 90% protection on the real problem areas and 80% on less problem areas. There are really four times we recommend 90%. First would be if the homeowner has single pane windows. Second would be if the homeowner gets more than 4 hours of "direct sunlight" on a window. Third would be if the homeowner has windows that do not have any interior sun control such as blinds or curtains. Fourth would be if the homeowner was getting a lot of glare on a computer screen or TV screen. Other than that, 80% works just fine.