Once you have the reflector complete mylar would be a pain to have to clean. As soon as a surface gets dirty/dusty its reflectivity drops dramatically and mylar doesn't enjoy being cleaned as it will crinkle and tear. I'd say paint it flat white and if you want to add mylar later down the line then you could simply install it over the white paint.

Flat white is a good choice, since it's quite reflective and also has a nice appearance. A reflective fabric called Foylon is similar to mylar but tougher and easy to wash. I made up a room divider lined with it and it works great.

Any material, whether mylar, foylon, or something else, should be at least a couple of feet from an HID light, and a foot from a fluorescent. If you're building a reflector that will be close to a bulb, make or buy one of metal. Specular aluminum is what light manufacturers use.

White is perfect for something like a light hood. Remember you are just trying to bounce all the light in the general direction of the tank, not focus it into a spot. Use a suitably shaped reflective material if you need to get very directed light from a lamp mounted far from the plants, for example to achieve high penetration on tall plants.