I've used 150's over a 75 before and to be honest wasn't too happy with the spread or intesity I got from it. There were "dim" spots to either side, and in the middle. The fixture was a Current 2x150w MH w/ 2 130w PC actinics. Not a cheap fixture by any means, but not top of the line either. If you were planning on doing the clamp on 150's I would think you would get even less spread, from what I have heard they don't have the best reflectors on them. Be wary, in lighting the old saying "you get what you pay for" is extremely true. Not all ballasts or bulbs are created equal. even if they are stamped 150 watt electronic ballast, or 14k 150 watt bulb, they differ from one to another. Take your time, save up a few extra bucks for a good fixture, it could save you alot of money in the long run especially if you buy a cheap one now...just to go out and buy another later because you were not happy.