I rarely use either one and have perfectly healthy teeth and gums. I believe in cleaning from the inside rather than the outside. A diet, like one rich in fruits and veggies, that rids the body of toxins and supplies nutrients to your gums and teeth so they can fight off disease all on their own.
Mouth wash is much better in killing bacteria and more effective. Warm salt water is good when you have some bleeding and nice to have cleansing.
I think both saltwater and mouth wash work, however, I would think that saltwater would be less effective as it's not specifically designed for rinsing the mouth.
I use a combination of both. Not together but I mean in the morning I will use mouthwash and at night salt.