SELECTING TENDER CUTS
Relatively weak, low-activity muscles make for tender steaks, while stronger, high-activity muscles are better as slow-cooking and braising cuts.
When selecting tender cuts, always look for four things:
Fine meat grain
Fewer muscle groups within the cut (a single muscle is best)
Little connective tissue
Lean to medium amount of fat marbling (depends on your preference)
Tender meat should have very fine clusters of muscle fibers that feel soft to the touch.
Connective tissue makes tender steaks chewy
Choose tender cuts of meat with only one muscle, or as few muscles within the cut as possible. Multiple muscles basically indicate a greater amount of connective tissue, and the fibers of each muscle tend to run in different directions, together these factors make for a tougher steak.
Even cuts of meat that are tender can have unfavorable qualities. For example, the nerve end of the strip loin, located at the rear of the muscle from which New York strip steaks are cut, can sometimes have a very thick ribbon of connective tissue running into the meat, which is extremely chewy to eat.
This connective tissue is a good thing if you braise the meat, giving it the time and temperature it needs to break down into delicious gelatin. But when we briefly cook tender cuts at relatively low temperatures, there isn't time for the connective tissue to break down and become tender. So when purchasing a tender cut of meat from your butcher, always inspect the meat to make sure there is not unwanted connective tissue.
Choosing fatty or lean steaks
Meat with a lot of visible fat (marbling) usually has a more robust, intense flavor than lean meat. It can be expensive (simply because fattening up a cow is a costly process), and it can also make for a chewy eating experience if not cooked properly. Some cooks think steak with a lot of marbling makes the meat more tender, but in fact, it's can be just the opposite: it can make the steak tough and leathery. Meat with extreme marbling is usually best when shaved thin (as in carpaccio), or chopped up (as in tartare). This way, the "chewing" process is already done for you, and you're free to enjoy the robust flavor of the well-marbled meat.
Inherently lean cuts (like a filet mignon, for example) are often very tender, but they tend to have a blander flavor profile. That's because muscles that work a bit harder in life will tend to accumulate more of the proteins and fat that contribute to the meaty flavor, at the expense of becoming less tender.
When shopping for steaks, it's often best to seek a happy medium between lean and fatty. Look for steaks with some marbling, but not completely riddled with fat throughout. Try filet mignon for tenderness, or ribeye or strip loin for a balance between tender and flavorful.
Expensive vs. inexpensive steaks: knowing what really matters
In the photo below, the steaks on the left are expensive, but very high-quality Kobe-style steaks from a local producer, these steaks would receive a USDA grade of prime; the steaks in the middle are less expensive USDA choice-grade steaks with average marbling; and the ones on the right are low-cost USDA select-grade steaks that are common at many grocery stores.
In general, it won't work to choose cuts based on brand, producer, grade, or price alone. It's always best to inspect the meat for desirable qualities instead. (That is, unless you're dedicated to the practices of a certain producer or company.)
For example, we'd rather eat the steak, on the top right, than the expensive Kobe-style steak on the bottom left. The steak on the top right, cut from the shoulder-side of the sirloin, is one uniform muscle, without any visible connective tissue. That steak, when cooked properly, will be tender, juicy, and flavorful. The steak on the bottom left is probably cut from the rump-side of the sirloin, and has thick connective tissue running right through the middle. If you cook that steak the same way as all the others, you'll have a chewy, gristly piece of meat, with only a few nice bites throughout.
However, the well-marbled steak on the top left will be super flavorful and fairly tender, whereas the generic steak on the bottom right will be tough and chewy because of the connective tissue running through it. As you can clearly see, just because a steak is expensive doesn't mean it will be delicious.