There is more to 'security' that just 'an' A/V. You need more to get complete protection, and it depends on the user, but a FireWall should be implemented as well, and both an in and out variety for complete coverage. Then you can think about Parental Controls and Personal safety, the sending out of SS#'s and ID's for instance (I find it works well but is a royal pain when you really want to send that data so I for one don't use it).
Next is 'footprint', how much RAM does it use AND does it matter to you? Some products are not good for low RAM systems and hinder system operation.
After that, the system load factor needs to be taken into account. Have a minimal system, this is very important.
Lastly, SERVICE, and if you have a problem, this is a big deal. Generally speaking, paid for versions have more, web sites, community help forums, and in some cases even free vendor service.
Dead last is basically integration, both in terms of suite's and on the desktop.