Can we start a case study of subdomain vs subdirectory? Start discussion...

a case study is very useful for learning something. I am very confused about subdirectory and subdomain. I cannot understand which I should use? what should I do? which will be better for me? thats why I want to start a case study here. so I am requesting to all experts, please start a case study here, so that everyone like me can be benefited from here. Can we start a case study of subdomain vs subdirectory? Start discussion...