The main difference is that Comertia provides a more active online marketing to it's PRO users. When you become PRO, you have an initial 20-minute interview to identify your business goals and the profile of your ideal client. This information helps Comertia purchasing online ads on external sites to actively drive traffic. In addition, your pages will be prioritized in related search results within Comertia.
In general, the use of Comertia for free is more passive because you will receive visits only from buyers who come from internet search engines. With the PRO subscription, you also receive visits from buyers who follow the advertisements that Comertia buys regularly.