Need to send a lot of mail from the server? 500,000+ Plesk due to support of Postfix.
Resource starved? Plesk
PCI Compliance? cPanel
Are you new to server administration? cPanel
SSL for multiple domains? Plesk
Highly configurable Apache/PHP/MySQL versions/settings? cPanel
Troubleshooting complexity? cPanel is easier. (Plesk stores everything in a cryptic database)
I’ve worked in webhosting for 5 years. I’ve found that Plesk is more similar to what you would expect of a standard CentOS installation with mail, mysql, and web servers all being installed via RPM. If you’re familiar with these configurations, go with Plesk but don’t expect much customization without disturbing Plesk’s functionality. cPanel, on the other hand, is highly configurable and includes many softwares that Plesk will not support.
Where I work, the majority of the staff are not fond of Plesk. It’s likely because many of them have never setup a standard Linux web server without a control panel. It’s also because when Plesk breaks, it breaks bad. Parallel’s support is also lack luster at best. I do like Plesk, not more or less than cPanel. I like that you can specify the PHP Handler for each account as opposed to cPanel that requires the Handler (SuPHP, Apache module, or Fcgi) to be set server wide.
cPanel is incredibly user friendly. cPanel/WHM are nearly unbreakable from within the panels. If you really try, you can break it from command line, but it’s usually pretty easy to fix. I’m a huge fan of the aftermarket support for the cPanel platform. People are constantly making new plugins and plugin development itself is relatively easy if you’re experienced. CSF seems to be more oriented toward cPanel and is highly configurable from within the WHM panel.
So what would I choose? If I were providing webhosting I would offer cPanel. I believe it would result in less support requests and allow the users an easy to use panel. If I were hosting my own sites, I would go with Plesk. I don’t run anything that requires PCI compliance and am usually more concerned with performance than anything. There is a huge learning curve switching from one to another, but it’s nice to have options.