I have never tried to work with Mylar however my experience with Panda Plastic has been nothing but great.
It is inexpensive at $1.50/ft - 1 ft yields 1 x 10 ft length.
It is easy to cut and wrinkles don't matter much.
It is washable and quite durable. (Try washing mylar with bleach after a grow and tell me how that works for you...hehe)
It is locally available and doesn't make people think, hmmmmm what does he need nine rolls of Mylar for?

Just my 2 cents based on a first time experience.