After reading the comparison tests in June PCA, I am asking the same question. At the moment I have the 30 day trial of Spysweeper and it's blocking items that my other spyware did not and I was going to buy the years protection, but Counterspy looks very atractive and cheaper.
I signed for Counterspy, due to the review and to the cost, if you buy 2 licences you get it cheaper and can run it on 2 computers, did that and split the cost with a friend, I also have Spybot and AdAware. The Microsoft one may be charged for later on and if so theres no details of costs, at least I know how much the Counterspy costs so I've chosen that. Which way to go? Don't know and can't advise but you can see what I've chosen to use.