You really need to do both. Air pushed in has to go somewhere, and back out the fan it came in usually is the only way (unless you have other vents). Air sucked out has to come from somewhere, so air will have to go in at some point.
If you have two fans (one in, one out), then it creates an air corridor, which moves the air in one direction only, making the two fans more effecient at their jobs.
If you can only put one fan in, though, I don't know which is better. It depends on what type of case (Midi Tower, Maxi Tower, Desktop, Laptop, etc) it is and what obstructions you have inside (an AGP card right next to the air intake, for example).