Hi, Philip:
I don't think you've stated why you're considering other solutions than the InDesign one you're already using. If you could state your satisfactions and dissatisfactions with ID it would be helpful to you, as well as to anyone who might have a suggestion or recommendation.
I looked at the comparison at http://www.adobe.com/products/framemaker/comparison.html
and found marketing-speak that clouds some of the information. There's also some comparison of features that aren't close-enough counterparts to be compared. Finally, some third-party tools are mentioned that supplement FM, but some that supplement ID aren't mentioned; for example, cross-references.
One example of an incomplete comparison is drawing tools. You can do a lot with FM's rudimentary drawing tools (they were advanced when first introduced, but they haven't evolved.) ID's drawing tools are Illustrator-class.
If your workflow uses external tools and simply places them into your documents, superiority or inferiority of the drawing tools are a non-issue. Similarly, other aspects of your workflow may indicate which features of which product should carry weight when deciding on the most appropriate tool.
RE: Thomas Bro's points:
* InDesign lacks FrameMaker's ability to apply master pages depending on body-page content; however, FM added this recently - in the past it was available as a third-party tool. ID's designed as a container of plug-ins; there are many third-party tools extend it, both plug-ins and scripts. ID's also designed to be scripted by relative amateurs as well as professional developers.
* There's a cross-reference plug-in for ID, from dtptools.com, that's a bit more advanced than FM's built-in. Like FM, they become links in PDF documents.
* Although ID currently lacks conditional text, XML manipulations can do the same thing. In fact, there are third-party XML tools for FM that claim to offer more conditional flexibility than FM itself.
(PS: Thomas, I read a long time ago about someone who wrote, as well as edited and reformatted, documentation in DOS's EDLIN, where each line ends with a carriage return, and almost all operations require special trigger keys to perform them! It's only one or two steps above cuneiform.<G>)
If you need to create online Help systems, FM's the clear choice for the simplest path through well-proven tools. XML from ID could be used to work with help tools, though it's not a a common path yet.
HTH
Regards,
Peter Gold
KnowHow ProServices