Hello,
We are in the process of the same comparison: Cisco vs O365 E5. We have been using our current Cisco ESA environment without to much issue the last 8 years. The main pro is that it is very customizable compared to Office365/Exchange. This gave us the chance to move legacy based environments to our new environment. Now we are looking at both solutions. But basically it is a matter of price who will be the winner: E5 has the advantage.
What is your consideration for looking at Cisco as a possible solution.