It seems that every time I go to research a site, I get conflicting data between Alexa and Ad Planner (the ones that I use the most). The most important metrics: age and gender, are most often totally contradictory from one another.
And then when I go to Quantcast for the tie breaker, they of course don't have the data.
If I had to guess who is right between Alexa and Google Ad Planner, I would think that Google would have more accurate data. But who knows...
Have you tried compete.com too?
The thing you have to bear in mind is that unless users specifically fill in personal details there is no way any of these sites can give you an accurate snap shot. According to Googles ads preference cookie I'm in my 40s!! (I'm 28 by the way). I quote;
"We infer your age and gender based on the websites you've visited."
So from that I read that they guess who I am by looking at the websites I visit then use the "demographic data" they have just made up about me to tell others who I am. it's all lies - making it little to no use at all if it's only guess work.
Personally, I'd say if none of the data matches on the various "demographic" tools, the only thing you can do is use the good old grey matter.