If you consider the total amount of light that hits your walls during a grow, it represents between 50% and 10% of available light, depending on plant height and the reflector used. Of that reflected light, this is a chart showing what percentage of light is reflected by different materials. If a flat white, high quality pure white paint is used, you can get as much as 93% reflectivity. The manufacturer can tell you the exact number for any given paint with one email. Mylar that is undamaged and pristine will reflect at best, 95%. This 2% difference is negligible in plant applications. Of course, this makes sense only if you use the proper white paint. The advantage of paint is it's ease of replacement, and ease of use and care. Keep in mind that all Mylars are not the same. Some Mylar, as shown by the below chart only reflect 90%. In that case, white paint would be superior.