The reason I said free is better is due to this blog post from OkCupid http://blog.okcupid.com/index.php/2010/04/07/why-you-should-... in which they say a lot of the profiles in pay sites were created for free or are expired which means a large percentage of the people you contact will be inactive/haven't paid

The way to make it obvious is through marketing ;). In my previous comment I tried to give some examples of how the paysites distinguish themselves from the free sites.