One of the most useful documents I've found when comparing help authoring tools is one by Matthew Ellison of Ellison Consulting, Ltd.: Comparison of HAT Paradigms and Workflows at http://www.writersua.com/ohc/suppmatl/suppmatl07/ellison_tools.pdf [1]

Why was it helpful? It examines:

• Tools available today
• Selection criteria
• Development paradigms the tools fit into
• Examples of each paradigm

First, it dissects the three main HAT paradigms and lays out a workflow for each. This describes the way your work flows, from beginning to end. It is essential to understand the workflow if you are to select the most useful HAT for your project.

  How can I understand my workflow? This topic does not appear as such in the article, but is essential to determining which tool will work best for you.

You'll need to develop a project needs assessment. It should answer these questions (5 W’s):

• Who will use the HAT to develop content? (Writers, authors, editors, contributors, possibly developers)
• Who will read the documentation? (Target audiences)
• How will they access the document? How tech-savvy is the audience?
• What is the source of the information? (e.g., MS Word, RoboHelp, as yet unwritten, HTML Help, etc.)
• Where and in what output format will it be published? (E.g., printed manual, online help, etc.)
• When is the manual or documentation needed?

Second, Ellison’s article nicely describes the advantages and disadvantages of each paradigm, and lists the capabilities of several tools in each paradigm.  

Finally, it suggests:

• Base your choice on the required development paradigm and processes
• Consider the primary output format
• Consider the need for topic-centered content reuse
• Test-drive several tools

I found these suggestions extremely helpful when selecting a HAT for my company’s sales training manual.

A section at the end lists helpful reviews of various widely used HATs.