Hi, I am new to ProcessWire too, but have experience with PHP programming and using other CMSes. First of all, there is no separation between
php+mysql vs php+processwire
Because it's PHP + ProcessWire + MySQL. PHP is a server side script programming language what ProcessWire is based on. To save data, it requires a database server and MySQL is the default one (but there others like Postgresql, MS SQL, Oracle, even file db like SQLite). ProcessWire is just one of the many content management system on the market.
The strength of ProcessWire is that it's easily themeable with any pre-made web templates, maybe that's why I couldn't find any frontend themes from the module directory
http://modules.processwire.com/
By contrast, you can find tons of pre-made themes for WordPress, Joomla, Drupal, Concrete 5, etc. So it is required to have some HTML, CSS and PHP basic knowledge to handle the template files in ProcessWire.
