Yeah, it's perhaps not a bad idea to have an antivirus, and check downloads and mail attachments with it. The only viruses we could get, as far as I can figure, would be MS Word macro viruses, which tend to work more or less cross-platform (unless their writers hardcode C:\WinNT\... into the things).
But, the point still stands, that comparing antivirus software for Mac relatively pointless. I guess the best antivirus software would be one you could configure to not automatically run every log-in, and to automatically check downloads and attachments.