well your plants will probably yield more with a 400w since ur focusing more on 3 bigger ones rather than many small ones, but two 250w will distribute the light more evenly among the plants (but remember 2 250w doesnt mean 500w worth of light, its just 250w of light over a broader area!) and if you do a mh/hps stagger with your 2 250s you will notice a much faster growth rate than with just one spectrum or the other. of course, the same could also be said with runnin a couple cfls with your 400w. theres so many ways to configure a space its really alot of personal preference, about like asking everyone what kind of car you should buy lol just whatever is practical for you and your needs. good luck!