Oh, gosh, where to start... lol. I have a few 40Bs, and have been asked this question a few times... 'what is the best light for a 40B?' Well, of everything out there, I like the 6x39wattT5 options the best. Better than my two tanks with 250wattDE pendants, better than a combo of the two. Its just a better option. And while you are at it, consider a 50B or even an Oceanic 58 (no cross brace). the extra height comes in handy with T5s on this tank.
IMO, dual 150s are a waste of money. you get more light from a single 250, and the 250watter will cost 1/2 as much for the ballast, reflecrtors, and bulbs. The distribution with the 150s is slightly better, but its not so big of a deal considering you can get a lumenarcDE or something for that single halide. the only drawback of the 250wattDE is that is mounts in the center, and therefore a centerbrace is not compatible.
T5s just give you more color-blending options, and, FWIW, a great deal more intensity where it counts. Most of my corals, even the high-light loving pink plating millipora I have which always seems to fade unless 5" right under th halide colors in better under the T5s. The only coral that doesnt like it as much is the large pipe-organ colony. Its getting too much light so I had to keep it under a rock. A 4x39wattT5 is enough, but a 6x39 gived you more flexibility with colors. T5s lend themselves to taller tanks though, and if I were to restart from scratch, I would redo the tank as a 50B or a 58 to give some of the lower light species a break.