Cost is not the only determinant between free and paid press release sites. The natural assumption might be that you get better results when you pay for them, but some free sites offer good exposure for your release; there is no guarantee a paid site is automatically better. Ultimately, performance is what you're after, and that can happen on either type of site. You might have to experiment to find which delivers more impressive results -- unless, of course, your bottom line is the determining factor.
Business Model
Broken down to basics, the difference between a paid press release website and a free site is that the former requires a payment to distribute a release through the website (and hopefully entails some value-added services in the process), while a free site is normally a no-frills way to post a press release without extras. Many free sites offer a paid upgrade option that allows you to access more preferred distribution channels.
SEO
One of the big differences between a free and paid press release service is the advantage gained in the area of search engine optimization (SEO) when you pony up a few bucks. Obviously, appearing in a particular place for search results is never guaranteed, but paid press release services usually offer more SEO-friendly options that allow the odds for higher placement to be tilted in your favor. For example, you can include images, live links, choose your keywords, or submit an RSS feed. Most free sites only allow the basic text to be posted.
Distribution
Not all, but some paid services have developed their own news network with journalists, industry contacts and relationships with other news networks. What this means to you is that there is a better chance of getting picked up as a legitimate news story and circulated around the Internet or even the hard-copy print world. This is how certain stories go viral and become the next big thing.
Pagerank
If you ignore everything else in this article and want only one thing to focus on to differentiate among press release websites, focus on each site's Google Pagerank. In this instance, a higher number is better. According to BrazenWebDesign.com, you should focus only on press release websites, paid or free, that have a pagerank of 4 or higher. Pagerank is an algorithm-based analysis Google performs on all websites to assign a relative "strength" score, or rank of importance on the Internet. One easy way to find the pagerank of any web page is to install the Google Toolbar, which requires the Firefox browser. Alternately, you could visit a third-party website like Page-Rank-Calculator.com and paste the URL of the page you're interested in there -- it will give you the pagerank instantly.
