Very simple distinction. Filet mignon — when served on its own — is usually butchered way to thick, especially at steakhouses. It comes out in a square cube and is a flabby mess. When left on the bone as part of a porterhouse — which is generally butchered a bit thinner — it cooks more evenly, has a better texture and has more flavor.
It can be the best or the worst. Only get it when prime grade and even then where it is cut off of the loin is all important. With a nickname like “veiny steak”, selection of the individual cut can be sublime or a third inedible.
Awesome and one of my go-to steaks. I buy a huge pack of Costco prime strips when I can get them and freeze them. Always a total people pleaser at my house. I had some Linz bone ins that I adored. I think this cut of steak might be my favorite with the bone in. (KC cut) A nice bone in ribeye being second. I can make tenderloin taste good with almost any grade. Costco Choice is hard to beat. Maybe Lafrieda Barrel Cuts or Bryan Flannery would change this perspective. My $0.02.