Try and look up wood supply houses in your area that cater to cabinet shops. The Borgs won't carry the good stuff. My favorite is Plumb Creek MDF II which is a very nice double grind MDF. Dense and heavy, but strong for MDF. There are other brands out there as well.
MDF lite is less dense than typical MDF which makes it lighter but not as strong. It will be the same thickness as other MDF of a given size. Often used for vertical/ceiling paneling and other low wear areas. Also for CNC tables since it allows more air flow.