Ok, I'll 'bite' ... am a 'fish' ...
My un-qualifications for this response: am not a true DB admin - just an interested community member with some experience with Moodles since 1.4.
Hopefully, someone from Moodle HQ might step forward and respond - or a Moodle Partner.
Here goes my 2 cents:
Google Search renders quite a few hits/pages.
https://www.google.com/search?q=MySQL+vs+MariaDB&ie=utf-8&oe=utf-8
Interesting ... MariaDB articles/pages from none other than MariaDB folks. ;)
Here's one that's 'independent':
https://seravo.fi/2015/10-reasons-to-migrate-to-mariadb-if-still-using-mysql
As I read the above page this one item verifies my experience with recently
assisting an entity migrate a Moodle 2.8 from Windows to RHEL7.
"As Linux distributions play a major role in software delivery, testing and quality assurance, the fact that the both RHEL 7 and SLES 12 ship with MariaDB instead of MySQL increases the likelihood that MariaDB is going to be better maintained both upstream and downstream in years to come."
Why is that significant? With CentOS 7 not so much as it's based on RHEL7 and
RH has always been 'conservative' (for lack of a better term) when it comes to
PHP and DB. Many times CentOS users have to resort to using reliable 3rd party repos (which is a debate in itself sometimes). However, having said that, RH and CentOS both do quality/stability testing before making such decisions. Trust your distro?
Item 5: Better performance
'... we at _____ migrated from MySQL to MariaDB, we saw moderate 3-5 % performance improvements in our real-life scenarios."
Now that should be the bottom line ... the one item that tips the decision in favor of MariaDB.
Item 3 of the above link:
More cutting edge features
Mentioned is GIS support.
Is there any Moodle code attempting to use GIS support?
Is there any roadmap for Moodle code to use GIS?
Is there anything in Moodle code that takes advantage of what MariaDB offers over MySQL/Other?
Some other items:
Migration might become difficult after 2015
and under that ... the real truth:
"One of our customers once expressed their interest in migrating from MySQL to MariaDB and wanted us to confirm whether MariaDB is bug-free."
Let me offer some more information:
http://db-engines.com/en/system/MariaDB%3BMySQL%3BPercona+Server
http://db-engines.com/en/ranking
I hope that:
"Looking for longest updates/longevity of installed software."
doesn't translate into 'install it once, and *try* to forget it!'!!! ;) We, those of that support the backend stuff, do desire stable and as little disruptive hickups as possible, I would think.
'spirit of sharing', Ken