Really the only way to test these things is to test them, but you can certainly find out publicly available information about how much traffic each site gets.
Alexa is the best known site for analysing market share, audience engagement, keywords, and traffic.
Punternet ranks 1,663,523 globally - that doesn't tell you how much actual traffic it gets, but this means that there are 1.6million websites which get more traffic. Alexa estimates that the average visitor only visits one page.
Punterlink has a much better ranking of 669,201, and it gets an average of 2 page views per visitor per day. Still not brilliant.
UKA does slightly better than Punterlink at 525,009 and gets 3 page views per visitor
Adultwork is obviously the big dog here. Ranking 47,010 globally. Now remember that AW doesn't even rank on Google for most search terms, so the majority of those visitors are just going straight there. Visitors view just under 10 pages each and spend 10 mins on the site, compared with under 2 minutes on the others above.
Like I said - these figures don't necessarily equate to bookings, they are simply traffic.