There are a number of tools out there which allow you to compose documents. Three of my favorites are TeX, Emacs org-mode, and OpenOffice. Each of these tools is open-source and allows the user to script and modify their experiences.

Below are some factors which I think are helpful to consider when choosing between these document-preparation tools.

Use TeX when:

1. You want output with very high quality appearance.
2. You want to take advantage of TeX's powerful layout algorithms and routines.
3. You want to typeset a bunch of mathematics.
4. You want the document to be version-controlled using Git or similar SCM system.

Use Emacs org-mode when:

1. Content is king, and you don't at this stage want custom layout.
2. You don't need access to the underlying layout engine.
3. You want to enter content, including mathematics, in a distraction-free and straightforward way.
4. You want easy export to LaTeX, PDF, and HTML.
5. You want the document to be version-controlled within Git or similar SCM system.

Use OpenOffice/LibreOffice when:

1. Ease of composition is more important than highly polished end-product.
2. You are content with fairly standard and simple layout conventions – and don't require pixel-perfect control or algorithmic layout optimization.
3. Mathematical typesetting is not that important to the document.
4. You need to edit the document in conjunction with other users who are not technical and do not know TeX.
5. You want integration with OpenOffice Calc (spreadsheet).
6. The document does not need to be in version control.