Framemaker is first and foremost a tool for building pages with. It never had any support for XML/SGML etc. in the earlier versions. It did have some robust automation features, and a structured way of working, which allowed it to be adapted to XML/SGML.
Arbortext started out as a tool for authoring content in SGML (and later XML). Somewhere along the way they bolted TeX on to it, driven by the FOSI language, and this allowed it to create printed outputs.
So the point is there is a philosophical difference between the designs of both systems. I believe that if you want to work with content, abstracted away from its actual presentation, then Arbortext is the best tool. This way your authors and editors are not thinking about how the pages look, but what the content means. This has real benefits when dealing with content that is being translated, profiled for usage (teacher vs student guides), delivered to multiple channels (EPUB vs HTML vs PDF), etc.
-Gareth