In my honest opinion I feel both are best. my reasoning is sometimes an ad on a free site can be upgraded to a paid ad to be a featured ad even though there are hundreds of free ads on that site having yours up on top so it is seen first can really improve the odds of the potential PSA to see it more often and we all know it takes anywhere from 5-10 pageviews of your ad before anyone clicks on it or not (some less some more).
But I have had the exact same results from both free and paid it is just a matter of preference and having a few extra dollars to buy the upgrade.
Personally I still prefer all my ads to be free because of my budget, but when I have a little extra cash I spend it on ad space on select sites that I know are high traffic sites that will produce the best results for my campaign.
so to summarize one is just as good as the other its just a matter of finding the right places to place your ad to get the best results
In my honest opinion I feel both are best. my reasoning is sometimes an ad on a free site can be upgraded to a paid ad to be a featured ad even though there are hundreds of free ads on that site having yours up on top so it is seen first can really improve the odds of the potential PSA to see it more often and we all know it takes anywhere from 5-10 pageviews of your ad before anyone clicks on it or not (some less some more).