Quantcast vs Urchin - Most reliable source?

[johnegood]

I'm looking at buying a website and have two sources of data on the site traffic.

Their urchin numbers which, for instance, list 100K sessions in a month.
The quantcast data (it's an uncertified site) which lists about 40K visits in a month. (I'm not talking unique visitors, I'm looking at visits)

Any views on which of these sources of data I should take more seriously? Any thoughts on why they might be so far apart?

[theapparatus]

Urchin as in the javascript from Google or the old Urchin that would read the server logs?

I feel going from server logs is best as you know those are correct. Folks can turn off javascript and they also don;t always load.

[seolion]

How visits are calculated usually differ from tool to tool, depending on the cookie time out. But certainly the difference is huge here.
Referring theapparatus here, see whether they are using self hosted Urchin software which reads sever logs. Thats more reliable.