I need to upgrade MySQL and I'm wondering if I should select MySQL 8.0 or MariaDB 10.3. Which is better?
Both MySQL 8.0 an MariaDB 10.3 are good, officially supported solutions for cPanel servers. From a purely cPanel point of view, neither is better than the other. The differences are in advanced features that are unlikely to matter for most cPanel servers - cPanel servers do not generally host applications that rely on the advanced and specific features of either database solution. If you have applications that you believe may require advanced features, please consult with a database administrator for a recommendation.
The main reason people use MySQL 8 appears to be that there is official support from third-party applications and vendors that do not always offer the same support for MariaDB.
The interface recommends MariaDB 10.3 as the default upgrade path. If you do not require advanced database features only available in MySQL 8 or get support from third party vendors that only support MySQL, selecting the default option of MariaDB 10.3 is a good choice.