I haven't used the FAT Fuzz Factory... but I can just say from my experience with the normal fuzz factory that it is more versatile than the wooly mammoth.
If you really REALLY like what the wooly mammoth does and how it sound... or you really need that sound then buy the wooly mammoth.
But if you are looking for a good gated fuzz with versatility go with the Fat Fuzz Factory, once you understand how the knobs interact with each other you'll be fascinated!!
LOOKING FOR A BJFE BLUEBERRY BASS OVERDRIVE (BBBOD) - IF YOU HAVE OR KNOW HOW TO FIND ONE, PLEASE LET ME KNOW.