Depends, some people don't have access to a MySQL database, so in that case SQLite3 would be better for them. On the other hand, for networks you'd much rather have MySQL support. I'd recommend add them both and make them configurable
The easiest example I can think of right now is from economy api, take a look: https://github.com/onebone/EconomyS...onebone/economyapi/provider/MySQLProvider.php
Oh, i actually chose yaml in that, do you know how i can convert its data to mysql (the ones which are already in yaml)?