Some manufacturers of AV products test for loads of viruses that don't actually exist in the wild - purely in labs. This makes their products have great looking stats for detection percentages and numbers. It's worth in the real world to you and me is precisely zero - the only test that matters is the wildlist test - viruses that actually exist in the wild and need to be stopped cold. So as a starting point when you are choosing an AV you should be making sure that it passed the wildlist test and from there going on to other considerations like heuristic analysis, update frequency, system footprint and ease of use.
The VB100 test is a good place to start which is done by http://www.virusbtn.com - you will need to register to see the results but if your AV manufacturers website doesn't say it passed the VB100 test you can be pretty sure it didn't as they are quick to put it up if it does.
Farkin Computers
In the pub