I get this question all the time. It really is simply the difference between how the t
wo products perform.
Moisturizer helps to hydrate the skin. It can help to plump up little lines and even keep moisture from evaporating from the skin too quickly. Moisturizer can make the skin feel soft and hydrated.
Primer literally creates a barrier between the skin and your makeup. It can help to fill in fine lines, and does not absorb into the skin. It usually has some form of silicone in it (look for ingredients near the top of the list that end in -cone). Primer creates a smooth surface for makeup to sit on top of, and not be absorbed into the skin. Primer leaves skin feeling silky to the touch.
I use both on clients. First, I will hydrate the skin with moisturizer, allowing a few minutes for it to "sink in". Then, I use a primer to allow for a
smooth surface for which to apply makeup products.
Can this be too much of a good
thing? Yes, sometimes the primer will cause makeup to
separate on the skin. Many makeup products now contain a form of silicone. Mix
moisturizer, primer and makeup with silicone (not to mention oily skin) and you have an overly slick surface that will just cause makeup to slip around too much.
I personally only use primer when I need to look as good as I can, not everyday. My eye
treatment does contain silicone, though.
Which primer should you buy? From my own trial and error, I think any primer with a form of silicone will work just as well as any other. So, if cost is an issue, go for the cheaper one.