Which sentence is more formal?

 - The surcoat, a type of outer garment with large arm holes, used to be cool in the fourteenth century.
 - The surcoat, a type of outer garment with large arm holes, was popular in the fourteenth century.
The first sentence is less formal. You can tell because it uses slang (cool).
The second sentence uses formal language in place of the slang, so it is more formal overall.