Webmin is free, and have many settings and features, but I haven't use it enough to tell more about it.
It's not a userfriendly control panel like cPanel or Plesk. But for a poweruser it could be a good control panel.
It depends on what your preferences are or what you want to do, if you want more lightweight I would say Kloxo, both are easy to install but Webmin is not really easy to use, so don't recommend this for new users.
Some say Kloxo is secure, but personally I had some bad issues with it, so Webmin wins on this one.
One available choice is to use both, or maybe get rid of them and get another one like DirectAdmin.