The only difference seems to be that you can pass a new grid as susy-breakpoint's second argument. Why not just always use susy-breakpoint and leave off the second argument?
Simply put: In which scenario(s) would you opt to use susy-media over susy-breakpoint?
Related Docs: http://susydocs.oddbird.net/en/latest/toolkit/#breakpoint
susy-media is our built-in media-query handling.susy-breakpoint was built to integrate with the popular Breakpoint plugin, before susy-media existed.If you have Breakpoint installed, then susy-breakpoint will call on their more complex and powerful media-query syntax. If you are not using Breakpoint, then susy-breakpoint calls susy-media, and you are really just using an alias.