After some reading, it appears that MariaDB could be a better challenger to MySQL and it runs, apparently, without any problems with CiviCRM (cf CiviCRM on MariaDB instead of mySQL).
But what about future compatibility? On the MariaDB FAQ they say
MariaDB 10 takes a different approach. While retaining close feature compatibility with MySQL, there are enough differences and enhancements so that it makes sense to start a new numbering scheme...
It's possible that in a near future, there will be too many differences between MariaDB and MySQL, and the compatibility will fall in the background. It is only a supposition.
But, will CiviCRM continue to develop only for MySQL, or could it be developed for both? Or (better for a 10-year user) for PostgreSQL? PostgreSQL, as a community driven project, is ethically closer to the Civi community than Oracle, I would think. (It's not a troll ;-)
Alongside that, some time ago, I had seen some posts about Doctrine and Civi. Has that integration gone forward?
In short: what DBMS would you advise for a new project with 200 000 contacts?