What is the difference between Atlantic and Steelhead farmed salmon in taste and how to cook?

Technically, steelhead is an ocean-going subspecies of rainbow trout (a term more commonly applied to fish who live only in fresh water). They're definitely related, though; salmon, trout, and char (you can often find Arctic char in the fish case alongside salmon) are all members of the Salmonidae family. As such they're pretty similar from a culinary perspective.

Personally, I find the flavor to be somewhat "fishier" and the texture less oily than Atlantic salmon, which is relatively mild. Steelhead is a bit closer (as the color would indicate) to Pacific salmon varieties, with coho being the closest match in my experience. But farmed fish are almost always milder, so don't expect anything close to really flavorful wild salmon.

The bigger difference is likely to be the physical size of the filets you get from steelhead. They are smaller fish than the quite large Atlantic salmon, which means a generally thinner filet and a finer grain. That in turn means they'll cook more quickly than thicker Atlantic filets.

Definitely an easy substitution, and not a bad difference in price (which is probably more due to where you're located in relation to the respective fish farms).

Steelhead is not salmon. Salmon Trout and Steelhead are types of trout, an entirely different fish from the same family of fish as salmon. A salmon is always a salmon, but a Steelhead starts its life out as a Rainbow Trout. If the Rainbow Trout migrates to the ocean, it becomes a Steelhead. If it never goes to the ocean, it stays a rainbow trout for its entire life. Because they are closely related, and taste similar, Steelhead is sometimes marketed as Steelhead Salmon. The store had it improperly labeled as such.

I find the two very close in flavor and would say it is probably a matter of taste. If offered one and told it was the other I would probably believe it. That is not the case with most wild caught salmon. The Steelhead (scientific name Oncorhynchus mykiss) is a rainbow trout (a type of salmon) that has gone to live in the ocean. They are from the same Genus as the salmon from the Pacific side (Sockeye salmon's scientific name is Oncorhynchus nerka). The Atlanic salmon (Salmo salar) is from another genus.

Make sure your Steelhead is farm raised in good conditions. Steelheads are between a threatened to endangered species. For some yet unknown reason, those raised in farms become domesticated and do not do well in the wild, so restoring them appears to be difficult. The farmed Steelheads also taste better in my opinion. Wild salmon gets its red and pink shades from a carotenoid in their krill based diet. The farmed salmon (including the steelheads) have carotenoids (canthaxanthin and astaxanthin) added to their diets, so color does not help much to determine quality when buying farm raised salmon, but it does help in the presentation.

Because of their lower fat content compared to other salmon, I would pan fry the filets (unless you have an unusually thick piece). You can follow the suggestions in this article at Serious Eats which goes through common problems arising while frying salmon.

The farmed Steelhead, in my appreciation of taste and cooking is tastier (less fishy) and not as dry as the Atlantic Salmon. Cook at highest heat (oven broil) don't cover, skin down, sprinkle steak spice, a little maple sirop, remove when golden and "WOW" just like candy.