I used to run a single 150 on my 24x24x12 frag tank and got horrible PAR readings. 6" down in the center, I was barely in the 200s. I was using a Magnetic ballast, Phoenix bulb, and a lumen max 3 reflector.
My vote would be for the 250
Mike...something had to be wrong with the setup. I just tested the par on my 150 Radiums, Aqua Medic Cube HQI Ballast and a LAIII Mini, the results were greatly different than you had. I tested two bulbs, the 20K Radium, and an old 10K XDE, in the pic below, the fist number is the Radium and the second number the XDE 10K, if there is only one number, it is the Radium. The pic is poor, I had to over expose it a little to get the back to show up for the text, my old camera isn't up to the task.
40x40x17" Pentagon, rock arranged in a 24-30" cove. Most corals reside around the perimeter of the cove. Most of the corals are 18-20" in height from the bulb.
Par at the glass shield was 1200 for the Radium and 2200 for the 10K
Par at the water surface, 9" from the bulb, Radium at 780 and the 10K at 1900. The purple Milli is directly under the bulb.
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To the OP, I would go with 250 watters, it leaves you with more options for tank upgrades in the future. I already have a 250 setup but in my case, I don't think I am going to need more PAR than my setup is giving me. Keep in mind that the ballast, bulb and reflector have a huge impact on what reaches the tank and how.
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