A key difference is in the development philosophy - MariaDB development is more open, and we (I work for the MariaDB Foundation, so feel free to do your own research!) welcome patches from the community and work to make this process smooth. You can read about Tencent's experience about not getting patches accepted into MySQL, and then submitting them to MariaDB, here. Here's another article highlighting some of the other differences - a key one is faster security updates, and if you're running a Linux distro, it's more likely to have MariaDB rather than MySQL as default.
Development in 10.3 is focusing on Oracle compatibility (as in Oracle RDBMS, not Oracle MySQL), so at the moment MariaDB is more compatible with Oracle than Oracle MySQL.