What is the difference between Seeking Alpha’s Marketplace and Seeking Alpha Essential(formerly PRO) or PRO+?

Seeking Alpha's Marketplace offers you the chance to get exclusive content from individual authors that complement those authors' free articles. If there's a particular marketplace author (or several) whose investing style or service suits you, you would likely benefit from their Marketplace service. And many users who sign up for one service end up finding several that suit their needs together. By signing up to an author's service, you get all their Essential articles as well.

Seeking Alpha Essential offers exclusive access to single-stock articles available on Seeking Alpha, to support stock research. 

PRO+  is a wider-spanning idea generation tool that gives you exclusive access to articles from a wide range of Seeking Alpha's top authors, as selected by our PRO+ editorial team.

Many users sign up to both Essential or PRO+ and Marketplace, as the services tend to complement one another for many users.