The Nars is called Skin Smoothing Face Prep and I have used it in one of my tutorial videos. It's not a liquid, it is more like a firm paste that I brush lightly over the skin with a foundation brush and it literally smooths out the surface of the skin and leaves it quite matt so that when you apply the foundation your skin looks flawless. I find it works really well on oily and mature age skin . It gives the skin a beautiful, soft look and is great for evening as it hardly needs any maintaining.
The Laura Mercier Foundation Primer does the opposite and when you put it on the face, after you have moisturised and before you apply the foundation ,it gives the skin an evenness and a dewiness that is noticeable. This is great on normal to dry skin and also mature age skin when you want the skin to glow. I have also used it on young women on its own without a foundation to give that fresh healthy look to the skin and then lightly dust over a powder like Mac Prep and Prime just down the T-zone. Gives a great fresh natural look.