The fsck(8) -T option is used to route options to the filesystem-specific programs it runs. The fsck(8) wrapper program gleans the name of the filesystem type from a variety of sources... on the commandline, from fstab, from geom info, etc. From some sources of info it decides a filesystem is ufs, from other sources it translates ufs to ffs.
This means that you can't reliably specify either -T ufs: or -T ffs: and be sure that the option will take effect. fsck(8) should probably translate everything (-T names and filesystem type names) to either ufs or ffs, but after poking around a bit in the code, it appears that this translation has to happen in many scattered locations.
A workaround is available, by specifying the option(s) you want twice, using both of the filesystem type names.