The next time you're in a super-mega-food-mart, try an experiment. Go over to the frozen fish section, and find some salmon fillets. If you have any luck at all, you should be able to find some that are marked "wild" and some that aren't - which generally means they've been farmed.
If you flip both over, and look at the nutritional information, you will hopefully see the amount of calories, protein, fat, and carbohydrates (probably not much of the latter, unless you've accidentally picked up a box of fish sticks, in which case put that down right now).
Usually, these days, the fat section of the nutritional information will include a breakdown of the Omega-3 and Omega-6 (O3 and O6) fatty acid content. Now, I'm going to really simplify this explanation, but basically, both kinds of fatty acid are used by your body. The problem is that our diets currently contain way too much O6 fatty acids (which can cause inflammation of tissue, among other things), and not nearly enough O3 fatty acids. The ideal ratio is at least 1-to-1, but most people get something like 1-to-40 (in favour of O6 fatty acids).
Okay, back to the fish at hand - not too cold, I hope? Anyway, look at the O3 and O6 fatty acid content of both the wild and farmed salmon. The wild salmon may have an O3-to-O6 ratio of 3.5 to 0.5, or 7-to-1 (this is an example, but pretty typical). The farmed salmon, on the other hand, may have an O3 to 06 ratio of something like 2 to 5, which (at 0.4-to-1) is much worse than the wild salmon.
Why is this? Well, to be blunt, it's what they feed the farmed salmon - grains. Wild salmon doesn't run into a whole lot of grain while it's swimming the ocean deep - it chows down on krill and such, and gets a healthy O3-O6 ratio naturally. Farmed salmon, on the other hand, hangs around the pen all day, eating "food pellets" which contain, among other things, grains.
The same is true of beef, to an extent. If you can find grass-fed, grass-finished (i.e. fed on grasses instead of grains right up until its last trip to the slaughterhouse), its O3 content is much better than typical grain-fed beef. Personally, I also find that the meat tastes better - almost sweet.
So, what I'm trying to say to you is don't just be concerned with what you eat - be concerned with what it ate!