The M&P 15T is not discontinued... the first gen that had a Troy quad rail is, but the second gen with the 1:8 twist melonited barrel, or corrosion protective whatever S&W calls it now, is still available to my knowledge.
Product: Model M&P15T - 5.56 mm, Tactical
Out of your list, the Sport is the best value, if you can live with the A2 front sight post. If not, then the Colt 6920 takes the same hit.
Personally, I would take the Colt LE6940 off the list as a first AR. Monolithic upper means you cant change out the rail if you do not like it. I do not care for quad rails... they are older technology and earned the nickname "cheese grater". I like the slimmer handguards, and with key lock or M lock, you can still attach stuff to the rifle if you are so inclined. If it is your first AR, you don't really know what you would like or wouldn't like.
Basically, between Colt and S&W, you will get more features on a S&W than you will a Colt, but you may not get all the "mil-spec" features like 4150 steel barrels, etc. You will have to decide what is more important to you.
I would also recommend looking at BCM. They have many configurations and you can probably get very close to exactly what you want from them.