They look the same because they followed the same principle in design: they were meant for urban environments (hence the heavy armor) and to destroy fortifications or dug-in troops. Since they both share the Sturm- prefix, it goes to show that they were designed with similar intentions. Just the same as a Panther was originally known as the Panzer V or the Tiger as a Panzer VI. They have similar roles, but built on different chassis with different weapons. Casemate designs were very popular among the Germans.