Charles Rowe shares four reasons/benefits he's come up with that should make you think about choosing MySQLi over the normal MySQL libraries for PHP for your application.
There still seems to be a lot of confusion over the differences between the two extensions despite the length of time that mysqli has been in the wild. I wanted to briefly review the four major benefits of mysqli.
Here's the list
- Prepared Statements
- Secure MySQL connections
- Multi query
- Object Oriented Interface
He also includes a few more links to further information (besides the explanation for each of the topics listed above) including an article from the Zend Developer Zone and a tutorial covering making the switch to MySQLi.