An index  workspace is a workspace  that is dedicated for indexing external
content sources. It may not be used as a document repository. A standard
workspace is mainly used as a document repository (along with its portal
interface). So if your intention is to set up a document repository (and use the
document management features of Sharepoint), configure your workspace as a
standard (not an index) workspace; if you intend to index external content
sources, you must configure your workspace as an index workspace, but I would
recommend you read  about it first because it is not that simple (read mainly
about indexing and crawling, and propagation of indexes).