AFAIK M+S only means that a certain tread/void ratio exists. Nothing to do with rubber compounds, tread design, or siping, which actually mean something in winter. This lack of useful meaning was the basis for the new RMA rating and the snowflake symbol. Almost all of the true winter tires now have the symbol. The only exeption I can think of is Continental. They have a couple of winter models that may or may not have it.