For me it was the cost factor. I really didn't think the 5 speed would fail, given the light weight of the vehicle and the fact that I only will have two wheels in traction.
But, to add a LSD to the 5 speed, I figured why not go ahead and spend about the same amount of money and get a 6 speed that was stronger and already has a LSD.
The 80 extra pounds doesn't really concern me, given the power I'll have at the wheels. Plus the fact that at 200lbs, I will act as a counterbalance being seated on just the other side of the engine.