The general consensus of the Oracle Tools Developers Users Group is that the ORCL Toolbox FormsTool diff ability is the best and brightest. Well worth the cost of a license if you have a diff need for form.
The problem with the FMB->FMT is that the queries in the forms are converted to a binary format which is unreadable by humans.
Forms version 10 has an ability to convert FMB to XML files, assuming you can read the XML files and do diffs.
Don't assume that just because you are using two versions of forms, the the underlying FMB files need to change. I've taken FMB files for Forms 4.5, and run them through the compiler for the Forms 4.5, 6i and 9i and gotten all of them to work just fine.
Which versions of Forms are you using?