So I am more of a Postgres guy so take this for what it is worth. I haven't spent a lot of time working with either MySQL (and when I have, I have been deeply frustrated, nor have I spent a lot of time with MariaDB either.

However, I do go to conferences and occasionally meet MySQL representatives and MariaDB community folks. This year I had some very long conversations with Daniel Black at FossAsia. I have also met a few others in the MariaDB community.

My impression from all the conversations I have had is that MariaDB is a far more healthy community than MySQL, that they are aware of where they have competitive edges and shortcomings and are interested in working on the latter, and they seem well ahead of MySQL at addressing these. While I plan to stick with PostgreSQL, if I was making the decision today, I would definitely switch. Community and openness to critique beats Oracle Corporation every time.