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).