The other thing to bear in mind of course is, the user experience.
Times are changing and reviews / tests are now starting to look at this, but historically AV products were gauged solely on detection rates. The fact that it might well have ground your PC to a halt in the process has been roundly ignored, not least by vendors. Symantec (Norton) is a poster-boy here; it always used to top the detection charts, but out of the box it was a heavyweight application with unnecessarily aggressive default settings which just caned the life out of machines. I hold the unpopular opinion that it's a very good product, it just needs setting up properly.
Some products are very chatty. They pop up so many largely unimportant questions ("MSN Messenger is attempting to access the Internet!! Allow this? [yes/no]") or ones that are impenetrable to most people ("System32.dll wants to access the Internet...") that people stop reading what it says and just 'yes' the box. Which is great until it actually finds something you do need to block.
Many are now "total Internet security" type products which pertain to protect you in all manner of ephemeral ways. More protection is better, right? The problem here is that a lot of what they do is unnecessary (you don't need another firewall when you've got two already, for a start) or can be negated by safe computing practices, as I touched on above. Education is preferable to reliance on fallible technology. If you get a virus because you opened an executable file emailed from email@example.com pertaining to be pictures of Brittney Spears naked, the issue here isn't AV failure, it's a lack of forethought (and poor taste in women).
Additionally, these 'total' products often cause more problems than they solve. By blocking network traffic and preventing from running any applications that they don't understand, they end up creating weird, hard to diagnose issues. VPNs, Citrix connections, messenger programs and remote support programs all commonly (and silently) fall victim to overzealous security products.
All of these points are part of why I usually champion MSE. It does what it says on the tin, and nothing else, with minimum fuss and minimum drain on system resources. I've yet to find anything that doesn't work as it should because MSE is installed; I can't say that of many others.