Too me Mylar is the best thing to have up its the most reflective material out there and it reflects all spectrum's of light well. THe only difference I know of between thickness is 2 mil is more durable so if you want to take it down clean it or lay a peice on the floor to reflect up it wont tear as easy with the shuffling of pots and evrything that goes on. THey both reflect just as well 1mil will just tear crink up and shit easier on ya. Its like 15 bucks a roll though for like 25 feet so its not bad at all, Theres a place in North Providence that sells it
Theres also another reflective material called Foylon its like mylar but more durable, I havent found anywhere around here that sells it though