Property portal watch posts on the difference between the two companies who provide traffic data and rankings for the industry. Hitwise and Comscore.
This has been written about before and much has been said about some of their individual nuances (especially Comscore).
I have worked with Hitwise which I have to say is an essential tool in any online marketeers toolbox, but you need a large budget to be able to pay for it. To this end, I would also have to say that I would rely more on a Hitwise ranking than Comscore. So when portal x is shouting that they are number 1 in the Comscore rankings, it is at the very least worth seeing if you can obtain the Hitwise rankings.
You can see the huge differences here (USA, both released within a week of each other).
10. NCI Interactive