Millimeters per second (mm/s)
Millimeters per second (mm/s) is a unit of speed. One meter is equivalent to 1000 millimeters and this unit is the number of millimeters travelled in one second.
Meters per second (m/s)
Meters per second (m/s) is the SI unit of speed or velocity. It represents the distance traveled in meters over one second.
It is used mainly in physics and engineering from describing the speed of a moving vehicle to the velocity of an object in free fall. It's simplicity makes it a good choice for describing speed and comparison with other units of speed.