Estibot Search Volume vs. Google

Many people depend on global search volume when making decisions around domain names. They look at these numbers when deciding to buy, sell, and when considering prices in some cases. A common question I see asked on forums is “Why is Estibot search statistics different from that of Google?” I have seen different answers, one recently which said that they are the same but it’s just a timing difference. That is not correct. To end the debate once and for all, I sent the question to Estibot’s customer service e-mail address. The reply was as follows.

Hi Mike,

We use proprietary data collection systems to estimate monthly search volumes. You are correct that the figures are not the same as Google’s. We are pretty confident that they are equally valid, though.

Thanks!
-Esa

So while this answer doesn’t reveal the exact source, we can now all agree that it is not the same as Google.   What has been your experience with these two different sources of search data?  Do you rely on one over the other?