I haven't had Bikeman or Aftermarket Assassins. On my Ranger NS, I ended up going to GGB, primarily because of the size, shape and placement of the muffler. It made it a whole lot easier servicing the transmission. The stock muffler was right in the way, and required using a long extension to remove and install the plug. It was always a pain to clean up the fluid afterwards. The GGB took care of that issue.
I contacted GGB back a few months ago and they told me they were waiting to get their hands on an Xpedition to do the R&D for a muffler. I'm waiting on them to release one. The Ranger muffler had a lot nicer tone than stock and wasn't really any louder.