I run 400 watt radiums with Galaxy ballasts on my 6 foot, 27" deep 150. I like a minimalist rock structure with plenty of open water at the top of the tank which means all my sps start at the halfway up mark. Because of this I went with 400 watters. I also have clams on the sand bed doing great.
I think that 250 watt bulbs when paired with the right ballast/bulb combination can be just as effective but with some caveats.
If I were to switch to 250 watt bulbs on my tank I would probably run a 10k bulb. After reading through Sanjay Joshi's lighting tests about ballast/bulb combinations it was clear that a 400 watt 20k XM on most ballasts won't have higher PAR than a 250 watt 10k XM. You have to look at all the variables.... ballast, bulb, reflector, turbidity, stable water parameters. Like others have already stated, there are a wide range of SPS corals requiring different amounts of light intensity. I feel that with a cooler looking bulb like 14 - 20k there is a larger margin for error in the 400 watt category. 10k 400 watt bulbs can pump out a lot of PAR and do a lot of damage if other parameters and light acclimation aren't stricly adhered to. Growth can be phenomenal with these warmer temperature bulbs, but it may not be the look you want as most of us like more blue.
Not that I've applied any science to what I myself have noticed, but here is my humble personal opinion....If you feed your fish a lot, then you shouldn't have the highest intensity bulb. My feeling and personal observation is that sps corals will slowly adapt to stronger lighting, but will then take in less food from the surrounding nutrients available, which may in turn cause excess nutrients and unwanted nuisance algae from the high feeding and high light. When lighting is moderate to borderline too little there may be another food requirement necessary to keep the corals thriving, which in turn can allow you to feed more. Again, this is just a theory based on experience in my own tank. So, to make a short story long....it depends. If you want to grow anything anywhere in your tank with a 250 watt bulb and it's a deep tank you may need to consider using a 10k bulb. If you want a more blue look but also want to grow sps near the sand bed like me you should look into 400 watt bulbs if your tank is deeper than 24 inches.
Hope I was clear.