Mcaffee or Symantec antivirus

If you have experience with Mcafee and trust the product then don't switch. If you feel it's time for a change then change to Symantic. I really don't like Norton's at all. I have had to many problems with it especially with the wireless you configure it you get it working then change one setting and it must be configured all over again. You may want to look into AVG from grisoft they offer a pretty good competitive package.

you can't use standard antivirus with an exchange server. The antivirus must be exchange aware or you'll corrupt your exchange installation

I currently run a large deployment of McAfee which uses Viruscan, Groupshield and E-Policy orchestrator across 2000+ desktops and servers (file&print + mail), as an enterprise solution it is a fantastic tool. Realtime alerts for virus outbreaks - management tools for control, installation, management and reporting from one central point are all excellent and very functional in wired, wirelless and remote environments.

I would recommend this system over pretty much anything else on the market as they have it nailed when it comes to large environments - although it works just as well in small environments.

As ever it comes down to choice. Ask yourself these questions:

1.How serious do you take your security?
2.Are you looking for centralised management - deployment - monitoring - alerts and reporting?

and of course -

3.What is the TCO of all the other systems (not just the simple cost to buy)?

Invariably well thought out enterprise solutions are more resilent and cost effective than point solutions which ultimatley will cost more.

NAI software has its reputation and we are currently using it too.
In comparison with Norton, I would go with Mcafee.

But the best that I could recommend is the SOPHOS Anti-Virus because of its network environment and system resource consideration.