While both ribs are delicious, they come from different parts of the pig. After the loin is removed, baby back ribs are cut from where the rib meets the spine. St. Louis-style ribs, however, are cut from the belly.  Baby back ribs are smaller, leaner and meaty while St. Louis-style ribs are well-marbled with fat. That marbling gives them such a wonderful flavor and helps them stay juicy and tender.