None of the tools will give you really accurate estimate. If they are your competitors by specific keywords, you can check their rankings, compare to yours and try to guess the traffic they get (you can use Google Adwords to see estimate searches for the keywords). But in general there is no sure way, everything is just approximation.
Send them an email, stating that you wish to advertise on their site and ask for information on traffic : PP
Negative, just checked on of my sites that is on sedo tracker and trafficestimate[dot]com is giving me my real uu x 4 so it must be counting page loads not uu