I too found that the 150 watt bulbs are a lot more expensive than the 300w. Then I looked inside the head and found a warning tab "Use only 300 watt bulbs". Inside my other head, same notice. Don't want to risk blowing the electronics - I have had one head conk out on me and pronounced unrepairable, so I think I need an ND filter.
I am not an electronics expert - can anyone comment as to whether a 150w bulb would endanger the circuit whilst 300w is safe? It occurs to me that if 150w is a problem, then the bulb blowing as it will one day must be even more of a threat to the electronics.