A document workspace is intended to be a site created for the purpose of collaborating around a single document or set of documents related to a specific topic. For example if your company was creating a proposal, you might create a document workspace for the tracking of documents, tasks and activities related to the preparation of that one document.
The document center is a template that could be used for a central respository of many documents for an organization. For example, a central documentation location for templates, policies, forms and corporate documents.[Stolen From Microsoft Site—Dont Mind]
IMP-The key thing to keep in mind is that no matter what template you select to create your site with, features such as versioning, workflow, tasks, permissions and folders will all be available to you.[Microsoft Says]