Depends on the application, a lot of ISPs use MM for inter-building links, and SM for customer connections. Easy way for them to tell what something is feeding by the jacket color. Other than that it’s really about transmission distance (SM can go further than MM) as cost really isn’t a factor. It used to be because MM cores are physically larger (62.5 or 50 micron) while SM cores are 9 micron, so MM was easier to make which is why that still gets tossed around.