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?

You better have CPU Power be hide that Unix Server, because installing a AV on a Mail Server will eat CPU Power and slow down Hard Drive Speeds. If you really need a AV I would say CA ETrust or Clam AV for the Unix Server.

Besides protecting a Windows Box is sometimes hard, just lock everything down in the AD so the clients cannot install crap or do anything - LoL.