What is imperial preference?
"Imperial preference" refers to "preferred", that is reduced, tariffs available to countries or colonies under British rule &/or members of the British Commonwealth as the Empire was later called.
The metric system is easier to learn and use, while the imperial system uses somewhat arbitrary numbers, such as 32 degrees for the freezing point of water. Which one you consider better all comes down to personal preference. It should be noted, however, that the United States is now the only country in the world still using the imperial measurement system.