Currently you use mod_php. mod_php is a apache module for PHP which runs the php processes under the apache user (www-data). This causes problems when you try to upload files with PHP as the apache user and web / FTP user are different.
With suphp, the php scripts will be run under the website user (which is the same then the FTP user). This is also mosre secure then using mod_php as every website runs php processes under different users then.
The main drawback is that suphp is a bit slower as php processes were started as cgi and that you can nout use settings like php_admin or php_flag in .htaccess files, in suphp you use custom php.ini files for that instead.