The sheen, or relative glossiness, of paint can dramatically alter the appearance of its color. While technically the color is the same, the amount of reflected light changes our perception. Glossy paint appears to be slightly darker than flat.
Flat paint is preferable for its soft, almost velvety appearance. It hides minor imperfections and can be touched up easily. However, flat paint has a strong magnetic attraction to greasy fingers, dust, and scuffs. The slightest brush can leave behind an enormous mark. Because flat paint can’t be washed, thoroughfares have to be regularly repainted. Many of the paint companies are now selling a premium matte paint that contains ceramic beads that make it washable, however, it is considerably more expensive.
Eggshell paint has a very slight sheen. It has the advantage of being less attractive to dirt and marring and can be lightly wiped. It does show a noticeable sheen difference when retouched, however.
Satin paint has a bit more sheen, and is washable. It is often recommended for bathrooms and kitchens. It cannot be retouched.
Semi Gloss is used for doors and trim. It is easy to wash. Semi Gloss is reflective, so surface imperfections are noticeable.
What do I recommend? It is hard to suggest anything other than flat for wall surfaces from a purely aesthetic perspective.
Flat paint looks better, because the lack of light reflection allows the color to serve its function: to be a backdrop to a design plan. You are not distracted by surface changes or blemishes. However, scuffed and fingerprinted walls are a decorating disaster, so it is probably best to use Eggshell in homes with small children and pets.
I never recommend Satin. In my opinion, people are unrealistic about how long they expect paint to last. The days of oil based paint curing on the walls and creating a glossy and impenetrable surface are over. Today’s latex paints are easy to apply and even easier to paint over, and should be done every five to seven years.