Personally, it depends on the situation! As Chaospaladin mentioned, mouthwash is good for bacteria but salt is good for healing. When there is damage inside my mouth I would prefer to use warm salt water, its a soothing substance that has been used for centuries. Mouthwash on the other hand, most of the time contains alcohol. I tend to stay away from direct bloodstream.
Personal mention, if I were to use both as mentioned above, I would do salt first then mouthwash. The salt contains antibacterial agents and dries up liquids of course, relieving you of bacteria nonetheless. However when adding mouthwash afterwards, its like cleaning something and then polishing it! Getting the rest of the bacteria out while giving your breath a soothing smell.
I only use alcohol free mouthwash, since it is not corrosive. The alcohol based mouthwashes can interfere with braces and similar.
Sea water helps prevent cavities
Using salt water mouthwash can be of great help to prevent cavities. Since, as we mentioned before, salt regulates the pH of the entire mouth. This prevents bacteria from forming cavities.