foil reflects light the same way a mirror does, directly, it dosent dispurse the light so you get hot spots and uneven lighting. mylar diffuses light so it bounces off evenly and in all directions.
mylar is clearly the better choice for growing
I got mylar loosely tacked to foam walls in my grow room. I've got some wrinkles and bulges and with the fans on the light flashes around like a 70's disco. So I think it gets some decent distribution.
