Remember we have to succeed in blocking every threat, and the bad guys only need to succeed once. I would say it is a mistake to only rely on one AV or AMP product to protect your network. With email being the most target entry into your network you will want to have all the protection you can get even if using the same engine.
A good illustration of why you should not rely on just a single AV to protect your network is the graph on http://www.senderbase.org/static/malware/#tab=0. Every malware vendor detects and protects against new threats at different speeds. Cisco does a pretty good job catching most of the threats, but every once in a while another engine detects a threat house and sometimes days before Cisco does.
Also AMP in E-mail will probably react different then AMP on network or endpoint. URL filtering is now included on ESA where it previously required the web security appliance. If you want bullet proof, or as close as it get you will keep you ESA/Ironport with AMP and two AVs. That way most threats will be detected one of the security layers.
You always have a decision.
