Aluminum foil works great, I use the shiny side, but it's not a big deal which side you use. I just find the duller side easier to adhere with glue to whatever surface need be, usually cardboard, so they're mobile without getting fucked up.
Flat/matte white paint is a completely diffuse reflector(also called Lambertian). Regular Al foil is about equally specular as it is diffuse. Aluminized Mylar is more specular than diffuse, and a mirror is pretty much entirely specular, with nearly zero diffusion.
Specular reflection is often called 'mirror image' or 'solid' reflection. Lasers use mirrors, and this keeps the beam nearly 100% solid, and like a mirror, nearly perfect reflection. Mylar would be close to a mirror, al foil would be noticeably more diffuse, and if you try to shine a laser off white paint... you'll probably will notice a very slight, very even glow(if anything). There is very little distance white paint will 'throw' light. Al foil is better, mylar even better yet, and a silver mirror reflecting a laser would be unnoticeable, as in you couldn't tell the original beam with a reflected beam.
Specular reflectors have the capability to create hot spots the easiest. As, like a laser and mirror, the light is being directed in solid units, according to the law of reflectivity. It's not being diffused much, and this can effectively double, triple, or quadruple, etc... your light's measured intensity, in lux, at the plant's surface.
Diffuse reflectors waste a substantial amount of light. They send off light in basically every single direction. It's not directed in any one particular direction like specular/mirror reflectors. Which is why it's basically impossible to create a radiant hot spot with a diffuse reflector.
That by no means makes paint a poor reflector(it's still roughly half as effective, a specular overcoat would help significantly, like enamels and powder coats). It(basic flat white) just can not throw light as well or as intense as a specular reflector. No way, no how.