The advantage of the more expensive product would be in the toughness. Trying to stretch and staple the thin, food grade foil on the underside of rafters without tearing would be enough to try the patience of most people.

I would put it on the underside of the roof decking just like the techshield..

That wouldn't work. You need an air space between the foil and roof surface or otherwise heat conduction will replace radiation reflection. They are two different heat transfer mechanisms and foil only works with radiation reflection.

That's assuming you are trying to keep heat out, of course. If you are trying to keep heat in it would work, although I don't know why you would use this method over standard ceiling insulation as insulation is much better for that application.