I'd recommend not getting the cheap Mylar, the thin 2mil stuff or even the blankets, if you hold a flashlight up to them you will see that light still penetrates the material, if you choose to go with Mylar I'd suggest getting the 6mil diamond print Mylar, the thinner Mylar is a waste of energy and materials for growing purposes imo, even if it says horticultural grade Mylar on the box, only around 90% or less light was reflected back out in my tests I conducted, plus I found any small scratches and wear in the material were easily made and allowed for even more light penetration through the material afterwards.
Id probably go with white paint as others have suggested if I were you, I don't know if flat or glossy is better personally but bright white seems to be the key factor to having enhanced reflective abilities found in the paint of your choice.
Either or will do the job, just depends on what you have to spend on it and the look you are going for, in a pinch paint spreads a lot further and is easier to utilize than roll or sheet would cover.