My company decided to use Nagios XI instead of Nagios Core. Nagios Core is open source and can be used for free. Nagios XI is not a free tool, which means you have to pay for the license. And the price is determined by the hosts/nodes you intend to monitor.
The Nagios XI is actually built on top Nagios Core, and also utilizes some other Linux tools. Its core components include: Nagios Core, Nagios Plugins, PNP, NDOUtils, NagiosQL, MRTG and Cacti.
The main purpose of developing Nagios XI I think is to make Nagios more powerful and less difficult to use. You need some Linux knowledge if you want to set up a Nagios core server from scratch. However, with Nagios XI you almost don’t need to type any Linux command lines. 99% work can be done through the Nagios XI web interface. Besides that, Nagios XI has some pre-defined functions which make it more user friendly, like monitor wizard, customized dash board.
There are couple ways to deploy a Nagios XI box. If you want to make your life easier, I recommend to use the VMware virtual applicance.