Flavor profiles of Steelhead Trout versus Artic Char
Hard for me to differentiate in words, but Arctic Char should taste about closest to brook trout which for the US would be its common relative.  Taste though will vary greatly according to wild/farmed, diet and water temperature.  As the name implies, an AC is a cold water fish naturally, and a wild one or even one that is farmed in its native range will typically have very red/orange flesh tending towards translucent. The color can to some extent be faked by farmers using dyes in the feed though as is done with farmed salmon and steelhead.  If farmed, to my personal taste, it is still better tasting than most farmed trout, in part because it often comes from colder water farms.

Steelhead is indeed a rainbow trout.  The differentiation between them is naturally, a steelhead is the anadromous from of the rainbow, that is strains that evolved to be born in rivers and then go to the ocean to live and come back to the rivers to spawn like a salmon.  Many though, including many location fishing regulations, simply define it as a rainbow under a certain size, and steelhead over that size.  They have been transplanted all over the world and in many places are steelhead if they originally came from steelhead rivers.  In large lake systems they will often use the lake as if it is their ocean.  Farmed steelhead differ from rainbow trout only in size if raised in fresh water.  Those raised in cold waters and especially salt water will tend towards more orange or red flesh, but it really depends on diet and again, can be faked.  They flavor of a farmed one tends to be similar to a farmed salmon.  

Anecdotally, my experience is that the farmed steelhead I usually find is higher quality than farmed salmon and a cleaner, fresher taste.  I speculate that they are from a better sourced farm with cleaner water and more access to crustaceans and less reliance on manufactured feeds, but that is speculation and personal experience and may be far from universal.  The flavor is consistently much higher than that of farmed rainbows that often have white flesh and are consistently fed commercial feeds in my experience.  I have rarely found char for sale, so most of what I have had was wild Alaskan char which I would put as very comparable to cold water brook trout which is by far my favorite trout (really a char).  A very clean taste, firm (for trout/salmon) flesh and bright color.  

Any fish that is not orange or red, or if that color does not tend towards translucent is likely farmed, fed a commercial feed, and may be from warmer waters.  Either fish is high quality if wild or raised in near wild conditions but will have differences that are difficult to describe.  Each person would have their preference according to their own tastes though.