Answer
The main advantage of metric system over English system is the uniformity among the different measures and it is not complex like English system.
Work Step by Step
Under Metric system, there are uniform measurement terms which are differentiated by some prefixes. Moreover, the relationship in series is also same. This is easy to use, learn and most importantly easy to convert. For example, to convert from kilometer to decameter there is just a need to shift the decimal and no further calculation or other formula is required.
However, in case of English system, there are heterogenic terms like inches, gallons, miles etc. The relationship between these units is also complex which need to be learned. For the conversion purpose, dimensional analyses are used under English system which requires a formation of the unit fraction to be multiplied by the given measure to get the desired unit.