In the installation phase may wonder: what is better? A MySQL database or MySQLi?
MySQL is the database so far, by default, Joomla! able to use in the installation of CMS, however, we are asked the type of database, the options offered MySQL and MySQLi.
What does MySQL?
A database MySQL "standard", with libraries and extensions to PHP from database for this, the choice of 99% of users.
What does MySQLi?
Contrary to what many believe, MySQLi is not a database type other than MySQL, but simply an extension for PHP that allows it to connect to the database (always and everywhere) MySQL saving a number of resources (improved execution time of query, table locks and so on).
What can I then use?
Usually all hosting offer MySQLi enabled already (particularly if the damage exceeds normal PHP 5.2), so you could install Joomla to work with MySQLi: however a check with your hosting can only clarify all your doubts.
What should I then use?
MySQLi is composed of a set of functions that nominally would be very beneficial to our database, if that was not on sites that do not generate a huge number of queries (in Joomla thousands of unique visitors daily running at least 4 / 5 pages of the site) so that it is quite uncomfortable bind to an extension that - probably - in the future will be very little support: in fact the project MySQLi, which was significantly enriched (and improved) functions to interface with the MySQL database, is considered almost a total failure, and we'll see - maybe - improved PHP 6, if not abandoned.
So our advice is to keep MySQL during installation and, if you like, made some tests on the test sites to prove that, objectively, would be gained.