Answer: The SI system is easier and more consistent. If the Klingons in the next galaxy know the definitions of metric, they can replicate the measures on their own planet. With English, they'd need to grab a yardstick the next time they did a crop circle.
Explanation: I'll use "SI" for the International System of Units. There are several key benefits;
- Unlike the English system, there are base units that form a standard for various measures: length (meter), mass (kg), Time (sec), electric current (Ampere), temperature (Kelvin), amount of a substance (moles), and candela (luminous intensity).
- A system of prefixes allow the base unit to be easily changed to accomodate larger and smaller quantities. 1,000 grams is 1 kg. 1000 meters is a kilometer (km), etc. In English, it isn't so easy. An English unit of measure may read "1 mile, 34 yards, 2 feet, and 6 and 1/8 inches." In metric it would simply read 1698.5 meters, or 1.698.5 km. Imagine the difference in finding the area of a rectangle in English vs metric: Either 1) 2ft, 3 and 13/16" times 4ft, 8 and 1/16 inches, or 2) 1.833 meter by 2.877 meter.
- The metric system has standards that allow a high degree of calibration. Distance is calibrated to the speed of light and time is calibrated tio a vibrating atom, for example.