From the looks of things, you’ll need to register via the link you provided and download their package, which I presume will take you to a closed source package for download.
While I have never installed ClamAV, it’s open source and available from the repositories - so it has been tried & tested for Ubuntu. Unless you absolutely trust Sophos, it’s up to you which one suits your needs. I personally don’t see a need for anti-virus, unless it’s a Windows machine.
Wild viruses are incredibly rare for Linux. There was a good quote along the lines of this I can’t find the source to:
Why write a virus to knock out a few thousand desktops, where a few hundred servers could knock out an entire continent?
While old Windows habits may be hard to break, Linux is a different architecture built in a way that makes it more immune to threats. Human error would be the cause of a lot of incidents, if any.
Take a look at these to understand Linux security a bit better:
http://ubuntuforums.org/showthread.php?t=510812 → “The Windows Mindset” part.
Remember, no system is ever 100% free from viruses. Some systems were historically made to give the user full administrative control (*cough* MS-DOS/Windows)
Don’t forget Ubuntu MATE also has a firewall built in, but disabled by default. Why it’s disabled by default, see here: http://askubuntu.com/questions/22667/why-is-the-firewall-disabled-by-default