@Jasmine, developer_mode is not decided if you are running on nginx + supervisord, it is controlled by a key in your site_config.json, if it exists and is a true value, so you are in developer mode, since you are “Administrator”
The difference about bench start and nginx + supervisor from bench setup production is.
Nginx and Supervisor will run as daemon, forever, while do you need it running, or the server is alive, or you have, energy, disk etc.
bench start run threads that you can kill, and up easily while you change a file in backed, or are you doing core chages. It exists for you run your app, in your machine, or in a VM.
Nginx + Supervisor is for run in a long living server, usually the production server.
But I prefer a mix of both, only frappe-web running in bench start, and everything else runing in supervisor + nginx, since is more fast bench clear-cache instead of restart everything.