The main problem with the latest Symantec Corp AV is that it's system intensive, and combines with a stream of errors that Symantec is apparently not interested in fixing, has made many people move away.
From personal experience here at my work, Symantec is also not too good at detecting malware and other nasties.
I can't comment on McAfee, since I've never used it. From what I've heard though, it's also started to suffer from serious bloat.
Once again, I add my voice to the NOD32 crowd, well worth a look. Your clients are being stubborn, and will only realise their mistake once people start complaining how slow the computers have become after you install the AV from Symantec or McAfee.