400 is probally cheaper, also less hassle to put up and adjust as oppse to 2 seperate lights. depends on your space, 2 250 will probally cover more area....
For me I would choose one 400W. I think the advantage in light penetration is superior to two sources of less penetration.
But this is only my opinion.
Also, if I had to pick only one from among 400 600 or 1000 Watts....I would chose the 600 as my preference!