Colin, I rode the previous Marius without the center fin, it was plenty grippy. The new shape for 2011 has two small removable side fins, should be a little different ride. Perhaps give them both a shot.
Stephen, not sure if you are interested but I figured I'd put it out there. I've got a new in box 2010 Faction 138, going for a very good price. (sorry for the hijack folks, couldn't get a PM through)