Opera is a browser which works on Linux, Windows, and Mac desktops, it is also an app for Android, while Opera Mini is also a web browser, it is only a mobile app which is compatible with most phones, like, Windows, Android, and iOS. Though both of them are popular, each is used for different scenarios.
The Opera browser is preferable for use in a situation where there is good internet connectivity and frequent secure sites, while Opera Mini is better if in areas with limited connectivity or high data rate. Of recent, Slant community survey has placed Opera on the 3rd rank, while placing Opera Mini on the 6th rank, in a poll for “The Best Web Browsers for androids.”
They actually cited some reasons which include the ability to able to add filter lists and to use fast and inbuilt adblocker. Most users just install both of them and use them as required.