By type
/Controllers/Events/Models
By topic
/Comments/Posts/Users
By topic, then type
/Comments/Controllers/Events/Models
/Posts/Controllers/Events/Models
/Users/Controllers/Events/Models
Mixed
/Comments/Controllers/Events/Models/Posts/Users

"By topic, then type" would also be known as a modular structur. It's generally a good idea simply because if you design your solutions to be modular you'll be able to expand them easily, while keeping code organised. See things like Symfony's bundles for example.