Despite the fact that some people claim "hot spots" with foil, it workd just fine as long as you use the diffused side (as someone already pointed out. Mylar is okay, but hard to work with, as it clouds easily and gets dirty fast. If you use mylar, you pretty much need to clean the shit out of it between every grow, which is a pain in the ass cause it streaks too.
I recently took a grow class with Ed Rosenthal, and he turned me on to Omniflect. I haven't used anything else since then, the stuff is so great. It is durable and easy to work with, has no maintainence, never clouds, no hot spots, etc. Many online grow shops have it, its not hard to find, and is comparable in price to mylar.
Good luck!