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

Why was it helpful? It examines:

During my research on HATS for beginners like me, I found these great resources:

List of available HATs for help authors who use MS Word at:
http://www.mshelpwiki.com/wiki/tiki-index.php?page=AvailableTools

List of reviews on many of these HATS:
http://www.mshelpwiki.com/wiki/tiki-index.php?page=SoftwareReviews

I’ve also gotten good insight by emailing authors of software reviews. Many have responded within days and have been helpful.

Mostly however, I’ve found that trying the application is the best way to determine if it’s right for my projects and purposes.