Universal symbols for molar mass, molarity, and molality
According to IUPAC, the official name for molarity is amount of substance concentration, and its symbol is $c$. The symbol for molality is $m$ or $b$, and the symbol for molar mass is $M$.

what are the meaningful differences between molarity and molality?

Molality does not change with temperature because mass and chemical amount is temperature-independent. The amount of substance concentration (molarity) does because substances expand (mostly) with in increasing temperature. So for most applications where the temperature changes, molality is the better choice.
On the other hand, for room temperature applications where volumetric measurements are convenient, it is common to use molarity as the measure of concentrations.