When you have a fair amount of time, kick back and read the final report of the 2007 Web Analytics Shootout. It is an exhaustive report based on tests that compared 7 different analytics packages across 4 different sites. The performance, accuracy, and capabilities were each tested, but in my opinion, the accuracy data is what stands out the most.
If you are thinking about choosing an analytics package, don't make a move until you read this report. It will be an eye-opener.