PRTG Vs Firewall

Well, you don't need PRTG in addition to a firewall. If you want to monitor the network behind the firewall, then you can use PRTG :) What exactly is your question here?

The prospect just wanted a justification for a network monitoring tool to be able to convince the management to take up PRTG which we have proposed. Is there a write up on what are the different aspects of the network that can be comparable? What PRTG does that cyberoam cannot do

You (i.e. your management) is comparing apples and pears here. Monitoring your network and the appliances within and protecting them with a firewall are two completely different pair of shoes :)