2x250 is what I ran on my 3ft 50g. I'm surprised I didn't get a call from the cops about a grow op
It was a lot of light! But 3ft is right between needing 1 MH or 2 MH, so I opted for 2.
Try out your 150 HQI in the middle and see what it looks like, you can always upgrade later if you dont think it is enough light. Or like I said earlier, keep most of your corals in the middle 2 feet, and use some NO fluorescents to brighten up the ends if you think they are too dim.