HI @rahul.dhakan,
I looked at the features you're currently using on PFsense and I think you could switch quite easily, you're covered on all points of your list and the new Routing Policy interface allows you to manage MultiWan routing without any hassle.
NethServer make use of lightsquid instead of sarg, because it provides similar results, but it's faster and require less space on disk.
For the bandwidth Monitoring you can use ntopng that has interesting features specially on the live side, about the monitoring of firewall performances you can use collectd, both ntopng and collectd are present among the standard packets of nethserver.
If you want to use something that works with Nagios you should easily find something for CentOS 6.7.
I just want to point out one big difference beetween PFSense and NethServer:
while PFSense shows a lot of fields in the web UI and allows you to customize quite a lot of your configuration directly from web ui, NethServer instead has a simpler webUI interface that ask you almost just the strictly needed fields to make work your configuration.
Usually this kind of approach works well for all the majority of cases and is faster to configure.
For the cases not covered by the web ui you can still do anything you want using the console and the templates-custom approach.