To be honest... just looking at the two sites... ClamXav and Intego... ClamXav seems much less "the sky is falling" going in favor of the "these are the facts" approach. In fact, I've never read the ClamXav home page... but what they say is pretty much exactly what I would tell anyone asking about antivirus software on a Mac. Basically, I would have no problem quoting what ClamXav has on their site. I think quoting what Intego has on there site would come across as being a bit alarmist. I can certainly say if I quoted what is on the Intego site to someone asking about antivirus software for their Mac, I would instantly get tons of people responding. But hey, they're selling a product. I don't find what they say to be nearly as offensive as what some other companies try to do (namely the fake pop-ups telling you that your system is infected and you must download their app to fix it). I also like that ClamXav offers useful best practices suggestions... which is really the best way of avoiding getting nasties on your system.
ClamXav:
http://www.clamxav.com/about.php
Back in the days before OS X, the number of viruses which attacked Macintosh users totalled somewhere between about 60 and 80. Today, the number of viruses actively attacking OS X users is none. However, this doesn't mean we should get complacent about checking incoming email attachments or web downloads.
Firstly, there's no guarantee that we Mac users will continue to enjoy the status quo, especially as there is now a small handful of trojans for OS X, but more importantly, the majority of the computing world uses machines running MS Windows, for which an enormous quantity of viruses exist, so we must be vigilant in checking the files we pass on to our friends and colleagues etc.
For example, if you're a wise person and you've turned off MS Office's macro support then you're not going to notice that virus which is hiding inside this month's edition of Extreme Ironing.doc which your friend sent you. If you then forward that document to a less wise person who has not turned off the macro support, then you have most likely just sent him a shiny new Pandora's Box with a sign saying "Open this end"!
Flippancy aside, I'm sure you get the idea: check the file before opening and/or sending it on to someone else. This gives you the opportunity to avoid the file altogether or at least copy and paste any vital information into a new document and send that instead.
Don't forget, if you run VirtualPC or VirtualBox or use any other means to run MS Windows software on your Mac, you can still become infected within that environment and lose valuable data on your Mac. These programs will run almost any Windows application you tell it to - virus or no virus - it doesn't know the difference. Note that ClamXav can only protect you against malware which you download and check on the Mac, so you may still want to install other antivirus software within your Windows environment.
Way back in 2004, I was looking for a low cost solution for scanning emails, documents and downloads for viruses. The open-source ClamAV project seemed to fit the bill and I used it for a few months, but soon tired of using the command line to operate it, so I took it upon myself to develop a graphical front-end in my own spare time. ClamXav was the result.
Since then, numerous features have been requested and implemented, and ClamXav has gone from strength to strength. When ClamXav turned 5 years old in September 2009, I released an new version, ClamXav 2.0, built from the ground up with an entirely new interface. It remained as beta software for about 18 months before finally being unleashed.
I hope you like ClamXav and use it a lot and that it helps keep you and your friends/colleagues virus-free. Although I'm providing ClamXav for free, I am increasingly spending more and more time developing it and would really appreciate if you would consider making a donation please. All donations (no matter how small) are gratefully received and would be a huge incentive to help me work on future versions of ClamXav, but more importantly I rely on donations to pay for my bandwidth costs. Please consider it.
Intego:
http://www.intego.com/virusbarrier/
Only VirusBarrier X6 provides comprehensive protection from malware and network threats. VirusBarrier X6 is the only antivirus program for Mac that includes full anti-malware protection together with firewall, network protection, anti-phishing, anti-spyware features and more.
As malware changes, protection needs to be reinforced
While viruses, Trojan horses, worms and other forms of malware are a constant threat to computers of all kinds, cyber-criminals are turning to new techniques to infect and control your Mac. These new threats come silently, with little warning, over the Internet: in many cases, simply visiting a booby-trapped web page can compromise your Mac. Focusing on malware or network attacks alone is no longer sufficient to protect Macs from the dangers of the Internet.
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VirusBarrier X6, the latest version of Intego's network and malware security program, includes more than 100 new features. With multiple threat-detection techniques, improved methods of detection, combined detection protocols, proactive behavioral analysis, and a full range of defensive functions, VirusBarrier X6 protects Macs from all known network-based threats, as well as all known malware.