i've read that painting white better and that only use aluminum as last resort, iz not very reflective
well, given only those two choices, i would assume painting white is better... however, the real answer is neither. Get Mylar. It reflects light like nothing else ive ever seen. If there is a hydroponic or garden store in your area, they will most likely sell it. If not, i know for a fact it can be bought on the internet very easily as long as there's nothing sketchy about you getting a big poster sized roll-package in the mail.