M is your basic piping for almost anything water except recirc lines which should be type l. Type l is also used for refrigeration as long as its acr grade. Type k is used for gas line service. All can be soft or hard drawn dependent on what you need it for and ease of install. If you looking for water piping spend the extra and get type l. Will always last longer. And don't forget type l fittings and long sweep elbows. Will last a lot longer as well.