Virus scanner for mail and file servers

I have to set up a virus scanner for a mail server and two file servers. The candidates are Clam AV, McAfee, Trend Micro and, to a lesser extent, Kaspersky. Not because I'd prefer any of those but because the first three are the only ones that are officially supported by one of the products we will be using.

Now I'd like to avoid McAfee. My personal favorite would be Kaspersky because of those candidates I know it best.

My questions:
How do Clam and Trend Micro compare to Kaspersky? Which of those three would you use and why?
Is the mail/file server version of McAfee as shitty as the one for workstations?

clam would eat the less CPU power and slow the HD less.

is this extra layer really that important? who are you pissing off so much that you need to go to such lengths?