This is a question that we spend hours and weeks answering for customers. Here's some short information relative to what's been said to date. If you'd like more, please feel free to reach me offline.
They could not be more different.
FrameMaker is not a native XML editor. It CAN do XML, if you have implemented the DTD. To do DITA in FrameMaker, you also need to purchase the Leximation plug-in for each writer. If you want a custom DTD, Frame requires that you implement it in the FrameMaker custom programming language and deliver an EDD. Frame will not open just any XML document.
Arbortext is an XML editor and publishing system. Frame delivers PDF, but Styler (or PE) produces PDF and html, html help, rtf, DMP images, etc, etc, etc. You can drop any standard XML or SGML DTD or XML Schema into Arbortext and be able to author, publish, and deliver source material and output deliverables created to that document type without a lot of customizaiton and programming. In many cases -- none at all.
DITA was created on Arbortext Editor at IBM. It was the first editor to support DITA and Arbortext (now PTC) was the only vendor who was a charter member of the OASIS committee once IBM released DITA to OASIS.