Always go for the wild Salmon. The one on the left is wild Sockeye Salmon.
The farm raised Salmon (the one on the right) is fed an unnatural diet (pellets) that gives it its color. If the Salmon wasn't dyed, it would be the color of Tilapia. And, presumably no one would buy it. As you can tell by the color, it even looks sickly.
Be sure to go wild!