I'm still working out the numbers myself, but I currently vote for at least 10 and probably 12.
12 gives you fearlessness for longer than anything else. An errant basilisk shell drops on you and gets a guy, then you get to be fearless for that pin test. Fear of the darknessed while still hiding behind terrain? Still fearless. Assaulting a hive tyrant? The horror is passed auto. If your opponent is trying to spread shooting to force morale tests, he will have to kill 3 bikers. With the 'always on' cover save, that means hes gonna have to cause 6 wounds on T5, and then if he moves on, you'll be testing on a 9.
Now, its expensive, its not indestructable like a
PM unit, but its damn shooty, hard hitting and fast. Like Teek said, with anything orky, if you have to decide between big mobs and little mobs, just go with big.