Chris,

Both are completely different, perhaps the sales rep could have presented you with more information and application. Each offers a variety of services targeted to specific needs. I think one have to read more in depth on the NAC product. NAC seems an excellent solution not only for authentication authorization but other regulatory complience.

When you see your sales rep ask for more information/Demo.

ACS is more widely use as a central point for access control to network devices switches routers, one example is having acs to manage accounting and command authorization accross all network devices using acs as a tacacs server.Whereas NAC is more of central point of security to provide inspection on systems prior to gaining access your network by either through LAN or from outside,one example of these regulatory defined complience could be inspections could be virus definition checks prior to gaining lan access thus preventing local network access if system does not meet regularoty complience defined in NAC the access is denied. Another example could be connections of unknown local host etc.. so it seems NAC is a much more wide product that provides an internal end point security, not just authentication authorization like acs..ACS has been out there for a long time, NAC is rather new product.

NAC

http://www.cisco.com/en/US/netsol/ns466/networking_solutions_package.html

http://www.cisco.com/en/US/solutions/collateral/ns340/ns394/ns171/ns466/ns617/net_qanda0900aecd800fdd6f_ns466_Networking_Solutions_Q_and_A.html

ACS

http://www.cisco.com/en/US/products/sw/secursw/ps5338/index.html

Rgds

Jorge