Oil or Latex?
Latex paints are more environmentally friendly than oil based paints. With oil based paints there are paint fumes to contend with that can be particularly bothersome on interior painting jobs. There is also a byproduct of dirty paint thinner that requires a trip to your local toxic waste site. (We always get rid of this waste as part of our work.)
There are other factors as well: Latex primers and finishes have been improving tremendously over the years and are very durable. For instance a latex finish on an exterior painting job will stay elastic for years, where oil based paint dries to form a hard and more brittle film. Surfaces expand and contract with weather. While the latex paint will move with the surface, the oil will tend to crack sooner with this movement.
The advantage of using oil paints is that it will give a smoother look as it tends to layout better on the surface and so not show brush marks as much as latex. Also, as was already mentioned, oil dries hard so it tends to be more washable than latex.